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!

Start by tracing how this Babel plugin currently invokes PostCSS for CSS module resolution. Identify the option-handling and transformation entry points, then verify that additional PostCSS plugins can be configured without changing the existing styleName-to-className behavior.

Written by the indexing model from the issue text.