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/
The tests still pass after removing the `if` (and the `compact`). I also can't think of a case where line 12 would return a falsey value. Since we're linting the diff, there should always be a modified line with a corresponding line number. I think it's safe to remove.

```ruby
def violations(file)
  violations_per_line(file).map do |line_number, violations|
    modified_line = file.modified_line_at(line_number)
    messages = violations.map { |violation| violation["message"] }.uniq
    Violation.new(file.filename, modified_line, messages)
  end
end
```