Support for YUI Compressor / ycssmin

Hello, 

while using flour on WordPress theme development there was the problem the internal LESS compressor has no way to preserve comments and it doesn't compress as good as the ycssmin compressor which is a dependency for the LESS npm module.

My commit adds support for enabling yuicompress / ycssmin and contains a test for the new option. Maybe you want to merge it into cake-flour :)