Major refactor for V3

[manveru]

This is a rather big PR, I worked on it for several days but think it's ready for some public scrutiny now.

Biggest changes are:

• Completely new CLI that actually provides some documentation as well
• All commands are now subcommands, the old bundix behaviour now is bundix lockfile (this is still up for discussion, as it's gonna break a ton of scripts)
• It's possible to generate Gemfile and Gemfile.lock now, even for only specific groups.
• bundix init now uses mkShell
• default.nix now uses bundlerEnv because I love recursion. Should figure out a way to make bundlerApp work in this scenario as well.
• Optionally generate JSON gemsets. They aren't consumed by anything yet and it's a good chance to take a closer look at how to make the existing format a bit better.
• Added platform to the lockfile, this also needs testing to see how it behaves on windows/darwin.
• Way more test coverage and some fixed tests, check with COVERAGE=1 rake.

Please let me know what you think.