Add Rabl Template

When I first saw Espresso, I imagined that it would be an awesome lightweight solution to a simple RESTful JSON API. One problem, no Rabl support. So I emulated how you added Slim and viola. I know there's probably a better way than this:

When I open irb and require 'slim', Slim is indeed in Tilt.mappings and if I require 'rabl' and then Rabl.require! then Rabl shows up in Tilt.mappings as well.

Just something to look into.