Set default 'rel' values for 'prev' and 'next' snippets
Rename 'page_meta' et al. to 'frontmatter_meta' etc
Allow the setting of extra stylesheets in the sub-layout's frontmatter

This will make sense once I show you the new sub-layouts in future
changes.
Update Slate image CSS

Taken from
https://github.com/jsncostello/slate/commit/56f9ee5ae5666377b00f2adced57b481435d695f
Add static tutorial CSS

I have layouts for it as well, but I won't include them here as they
will only be used in some places.