Feature: make postcss configurable

I like to use postcss to transform css with certain plugins, and wouldn't want to have to give this up in order to use this plugin. Luckily, it already uses postcss, so it should be pretty trivial to expose an option to process with additional postcss plugins if needed. I'd be happy to pull request this in, just wanted to open this issue to bring it up and see if maintainers had any thoughts!