Make the PostCSS transforms configurable.

[geelen]

The FileSystemLoader passes through the list of plugins to the loader core, but that's not configurable here. I have absolutely no idea how browserify plugins work, but if that set of plugins is configurable then this becomes basically a 'postcssify' that supports :local (like https://github.com/geelen/jspm-loader-css or webpack's loader without module-mode), which would be pretty neat.

[joshgillies]

Support for this has just been released! Closing.