Making css processing a plugin

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](https://github.com/UnwarySage/cryogen-core/tree/post-css-support) 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.