Added support for Ruby haml

Sometimes I like to use the actual Ruby haml gem to compile static templates for Angular projects and such. It has a somewhat more forgiving parser than haml-js, particularly when it comes to dealing with long attribute lists, something that Angular is notorious for.

This pullreq adds support for shelling out to the `haml` command installed by the haml gem, if the `language` setting is set to `ruby`. I've also created some tests for this feature, doing a little refactoring of the existing test code in the process.