Move to more defined plugin paradigm

[jbodah]

Right now all of the plugins are nested in /pre_commit. There are a few issues with this:

1. We'll probably want to allow for plugins like RSpec to be called with any hook (e.g. pre-push)
2. More difficult to scale. The more plugins we add the bigger this repo gets. I can imagine some plugins having specific configuration
3. Want to support different environments (e.g. Minitest vs. RSpec)

Minitest has a very nice plugin system that works very similarly to how we load them here: https://github.com/stupied4ever/git-hooks/blob/master/lib/git-hooks.rb#L29

Minitest's plugin load: https://github.com/seattlerb/minitest/blob/master/lib/minitest.rb#L78

[stupied4ever]

I was plaining to do it some day, but for sure, this description will help in the future.