Add CoffeeScript support

* Use [CoffeeLint] to keep to a community-supported open source standard.
* Propose a `.hound/ruby.yml` and `.hound/coffee.json` naming convention.
* `.hound.yml` will still work (backwards compatible) but would now be
  considered legacy / deprecated.

[CoffeeLint]: http://www.coffeelint.org/
Perhaps the meaning of `modified_files` needs to be re-designated, or removed completely. `modified_files` only appears in `StyleChecker` as of right now, and it looks like it should be `commit_files` instead, since it's all of the files touched in the commit/pr. I'm just not sure `unremoved_files` is better, or descriptive. I'd like us to find a better business name that means "all the files that have green in them". Really, we want the "edited" and the "added" files. Some suggestions `relevant_files`, `changed_files`?