Added an option for for scss output style.

Adds an option for specifying which scss output style you wish to use. Having this option allows easy configuration in .emacs files and also on a per file basis like so:

// -*-  scss-output-directory: "../css/"; scss-output-style: "compressed" -*-

This can be handy during development since it allows you to easily use the nested or expanded styles for debugging and then switch back to compressed for sending changes to production.