Add config files

[jaedoucette]

This PR is to address Issue #2, by adding support for external configuration files.

The proposed support exactly mirrors the behavior of the flake8 linter:

• Config files are expected to conform to the ini format, with a [blocklint] section header containing key/value pairs of blocklint command line arguments, separated by '='.
• A global config in ~/.blocklint is checked first, then a local config in ./.blocklint, then setup.cfg, then tox.ini, so that tox.ini has highest priority.
• Command line arguments override any arguments derived from a configuration file.
• Tests are included which verify the desired prioritization, though they test only a single argument. Global config files were tested manually, but automated tests are not included because they would overwrite a developer's config.
• README is updated with instructions for configuration.

[troycomi]

Looks nice! Can you:

• [ ] Fix your whitespace so pre-commit passes
• [ ] Expand your tests to include one of the list options and a store_true option
• [ ] Update tox.ini of blocklint so the new test runs during CI
• [ ] Consider using getboolean for your parsing. You can add more options so store_true still passes.

You could add unit tests for the config parsing by extracting to a method that accepts a list of open files and use StringIO objects in the tests. Probably overkill though.

[jaedoucette]

@troycomi Done!

Sorry for the force push. I am unable to run your diff-based tests under my local setup because of the expansion braces, so I relied on the CI to do it.

[troycomi]

Great work, thank you!