Adding the install subcommand.

beRi install [options]
To model the install subcommand things will have to be taken slowly. Functionality to build into beRi:
System installs, User installs, project installs, other?
A file that is conceptually identical to requirements.txt file
- .toml file
- packrat's project.lock file
CRAN remote repository management with defaults
miniCRAN local repository management
editable installs (long term goals are good to have too)
The pip install --help displays the following:
Usage:
  pip install [options] <requirement specifier> [package-index-options] ...
  pip install [options] -r <requirements file> [package-index-options] ...
  pip install [options] [-e] <vcs project url> ...
  pip install [options] [-e] <local project path> ...
  pip install [options] <archive url/path> ...

Description:
  Install packages from:

  - PyPI (and other indexes) using requirement specifiers.
  - VCS project urls.
  - Local project directories.
  - Local or remote source archives.

  pip also supports installing from "requirements files", which provide
  an easy way to specify a whole environment to be installed.

Install Options:
  -c, --constraint <file>     Constrain versions using the given constraints
                              file. This option can be used multiple times.
  -e, --editable <path/url>   Install a project in editable mode (i.e.
                              setuptools "develop mode") from a local project
                              path or a VCS url.
  -r, --requirement <file>    Install from the given requirements file. This
                              option can be used multiple times.
  -b, --build <dir>           Directory to unpack packages into and build in.
  -t, --target <dir>          Install packages into <dir>. By default this
                              will not replace existing files/folders in
                              <dir>. Use --upgrade to replace existing
                              packages in <dir> with new versions.
  -d, --download <dir>        Download packages into <dir> instead of
                              installing them, regardless of what's already
                              installed.
  --src <dir>                 Directory to check out editable projects into.
                              The default in a virtualenv is "<venv
                              path>/src". The default for global installs is
                              "<current dir>/src".
  -U, --upgrade               Upgrade all specified packages to the newest
                              available version. The handling of dependencies
                              depends on the upgrade-strategy used.
  --upgrade-strategy <upgrade_strategy>
                              Determines how dependency upgrading should be
                              handled. "eager" - dependencies are upgraded
                              regardless of whether the currently installed
                              version satisfies the requirements of the
                              upgraded package(s). "only-if-needed" -  are
                              upgraded only when they do not satisfy the
                              requirements of the upgraded package(s).
  --force-reinstall           When upgrading, reinstall all packages even if
                              they are already up-to-date.
  -I, --ignore-installed      Ignore the installed packages (reinstalling
                              instead).
  --ignore-requires-python    Ignore the Requires-Python information.
  --no-deps                   Don't install package dependencies.
  --install-option <options>  Extra arguments to be supplied to the setup.py
                              install command (use like --install-option="--
                              install-scripts=/usr/local/bin"). Use multiple
                              --install-option options to pass multiple
                              options to setup.py install. If you are using an
                              option with a directory path, be sure to use
                              absolute path.
  --global-option <options>   Extra global options to be supplied to the
                              setup.py call before the install command.
  --user                      Install to the Python user install directory for
                              your platform. Typically ~/.local/, or
                              %APPDATA%\Python on Windows. (See the Python
                              documentation for site.USER_BASE for full
                              details.)
  --egg                       Install packages as eggs, not 'flat', like pip
                              normally does. This option is not about
                              installing *from* eggs. (WARNING: Because this
                              option overrides pip's normal install logic,
                              requirements files may not behave as expected.)
  --root <dir>                Install everything relative to this alternate
                              root directory.
  --prefix <dir>              Installation prefix where lib, bin and other
                              top-level folders are placed
  --compile                   Compile py files to pyc
  --no-compile                Do not compile py files to pyc
  --no-use-wheel              Do not Find and prefer wheel archives when
                              searching indexes and find-links locations.
                              DEPRECATED in favour of --no-binary.
  --no-binary <format_control>
                              Do not use binary packages. Can be supplied
                              multiple times, and each time adds to the
                              existing value. Accepts either :all: to disable
                              all binary packages, :none: to empty the set, or
                              one or more package names with commas between
                              them. Note that some packages are tricky to
                              compile and may fail to install when this option
                              is used on them.
  --only-binary <format_control>
                              Do not use source packages. Can be supplied
                              multiple times, and each time adds to the
                              existing value. Accepts either :all: to disable
                              all source packages, :none: to empty the set, or
                              one or more package names with commas between
                              them. Packages without binary distributions will
                              fail to install when this option is used on
                              them.
  --pre                       Include pre-release and development versions. By
                              default, pip only finds stable versions.
  --no-clean                  Don't clean up build directories.
  --require-hashes            Require a hash to check each requirement
                              against, for repeatable installs. This option is
                              implied when any package in a requirements file
                              has a --hash option.

Package Index Options (including deprecated options):
  -i, --index-url <url>       Base URL of Python Package Index (default
                              https://pypi.python.org/simple). This should
                              point to a repository compliant with PEP 503
                              (the simple repository API) or a local directory
                              laid out in the same format.
  --extra-index-url <url>     Extra URLs of package indexes to use in addition
                              to --index-url. Should follow the same rules as
                              --index-url.
  --no-index                  Ignore package index (only looking at --find-
                              links URLs instead).
  -f, --find-links <url>      If a url or path to an html file, then parse for
                              links to archives. If a local path or file://
                              url that's a directory, then look for archives
                              in the directory listing.
  --process-dependency-links  Enable the processing of dependency links.
datasnakes / beRi

Adding the install subcommand. #2

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