Allow postcss configuration options

[gherkz]

Rebasing of asset URLs is pretty important for the critical CSS, which is achieved with the postcss-url postcss plugin. If would be helpful to be able to configure postcss to allow, for example, an alternative parser such as postcss-safe-parser.

This is currently impossible, which makes generating critical CSS for pages with syntax errors very difficult. I supposed this behaviour might be desired when you have control over the source files. However, it makes using critical rather difficult when you don't.

Either way, it would be nice to be able to pass extra options to the .process call for the URL rebasing (and maybe the postcss postprocessing as well).