Support for postcss ProcessOptions

[CallumJHays]

Currently, I can't use this plugin with the syntax or parser options in postcss.config.js. There might be other options that I'm not aware of.

These could be loaded in with "postcss-load-config", but currently NextJS displays a warning as it does not pass these options to process() either. Another alternative would be to have these options in this plugins' options instead.

[CallumJHays]

I've since realized I only needed this because I was unnecessarily using sass-style comments in css. I'm not sure what other useful parsers / syntaxes / etc are out on the market but the PR I've opened should allow for any of those.

I'm wondering why nextjs explicitly doesn't support these options either, do they have their own postcss implementation?