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/
Here's a simplification of what @sgrif and @mcmire are proposing. This might be more readable, but does call `modified_line_at` in both iterations.
```ruby
violations_on_changed_lines = violations_per_line(file).select do |line_number, violations|
  file.modified_line_at(line_number)
end

violations_on_changed_lines.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
```