Fixes #176 by adding css-specific "optimize" option

Removed the dependency on requirejs's optimizeCss option and introduced a css-specific option which can disable optimization, defaulting to true.  I added this under requirejs's "config" option which allows module-specific options without polluting the "global" config thinking that's the least intrusive option going forward, but happy to change that if desired.

Let me know if anything is amiss.