CSS, generally

[davidtheclark]

Besides inlining critical CSS in the static build (#4), there are other considerations:

• Authors need to be able to designate page-specific CSS, as text and as URLs.
• Authors need to be able to designate site-wide CSS, as text and as URLs.
• While the URLs above code be external (e.g. the Assembly CDN URL), I think we'll ultimately pull it down, bundle it up, and serve it locally. This way we can exercise some more control for optimizations, like delaying the loading of the site-wide CSS file until rendering is complete (since it's unnecessary before you change pages, since all CSS for the page will be inlined). This means we need to have users specify CSS as URLs, not as <link> elements to add to the head.
• While we could include some CSS processor defaults (maybe just Autoprefixer), it'd also be nice to allow users to specify PostCSS plugins to add to pipeline.

[davidtheclark]

Trick about external CSS: all of the asset references inside it (images, fonts) are relative to its actual URL. Will have to transform those in case they are relative.