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/
We use the term `modified_files` in multiple places to mean "all the files in the pull request which were modified", which includes files that were removed. We only care about removing lines in files that have some green in them.