First cut at allowing custom CSS.

This should allow CSS to be picked up from the project root and on a per-document level.

- Project CSS lives in the tree root and has the name `settings.PROJECT_CSS` or `project.css`
- Document CSS lives in the document path and has the name `settings.<PREFIX>_CSS` or `<PREFIX>.css`

Currently empty <style></style> blocks are generated even if there is no CSS file.