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/
If it's not safe to remove, what @sgrif meant is this

``` ruby
def violations(file)
   violations_per_line(file).
     map do |line_number, violations|
       modified_line = file.modified_line_at(line_number)
       [line_number, violations, modified_line]
     end.
     select { |line_number, violations, modified_line| modified_line }.
     map do |line_number, violations, modified_line|
       messages = violations.map { |violation| violation["message"] }.uniq
       Violation.new(file.filename, modified_line, messages)
     end
  end
end
```

You may be able to split this up into multiple methods or something. It gets messy though.