Add option to make arborium syntax highlighting its own stylesheet

[MoritzBoehme]

Currently, the syntax highlighting is embedded into the site directly, increasing the page size unnecessarily.
It would be nice to load these stylesheets via a link tag to optimize initial page load times.
This would even make subsequent page loads faster, as now the highlighting stylesheet can be cached as well.

[fasterthanlime]

That sounds like a good idea. Needs a bit of plumbing but nothing too bad.