Added support for SASS using node-sass

Added support for SASS using node-sass. Currently I'm using the 'nested' output style because node-sass is outputting a blank css string if I use 'expanded' which I think would be preferred. I'll change this once I get that resolved.

NOTE: Could not get tests to run but included the test code anyway for future fixing.

See: https://github.com/codeparty/derby/issues/264