Add new "build" top-level command

[oubiwann]

This could be used for:

• build deps (this could also do a rebar get-deps)
• build tests
• build project
• build all

Also, if build was supported, other commands could use them. For example, the REPL commands could take additional options:

• lfetool repl lfe build
• lfetool repl lfe build deps
• lfetool repl lfe build projects
• lfetool repl lfe build all

This would provide users support for many different workflows.

[oubiwann]

Notes from duplicate issue #53:

Subcommands would specify what to compile:

• lfetool compile src - compile .lfe and .erl files in ./src
• lfetool compile deps - compile .lfe and .erl files in ./deps/*/src
• lfetool compile plugins - compile .lfe and .erl files in ./plugins and ./~.lfetool/plugins
• lfetool compile all - all of the above
• lfetool compile - alias for lfetool compile src? Or lfetool compile all?

Used in conjunction with

1. a dependency/package definition and
2. download capabilities, lfetool could compile these to a standard location.

Again, this would mean people wouldn't have to depend upon rebar; they'd only need one tool.

We could, of course, also provide support for rebar, we just wouldn't have to require it.