Making css processing a plugin

[UnwarySage]

Hello.
I wanted to use post-css in my blog with Cryogen, and looked into #27 as a guide to how to do it.
I have a initial prototype of css rendering being a plugin, but would like input if possible.

the namespace cryogen_core.css contains the slot for a css plugin to be registered, and a protocol for them roughly based on the one from `cryogen_core.markup.

I modified cryogen_core.compiler to call the processor, but don't fully trust my grokking of the compilation process.

My next steps, absent central flaws, is making an actual plugin for sass, and figuring out the development environment setup for that.

[holyjak]

@UnwarySage if you send it as a draft PR then I can comment