Move slate-specific includes to subfolder
Only include Pygments CSS if it is enabled in the configs

Actually, GitHub pages forces Pygments to be enabled, however, it's
better if I only load the CSS for sites that use it, so we'll just
require myself to enable it in the configs if I want to use it.
Move slate-specific images to `stylesheets` directory

No more absolute URLs! Also, the `images` folder is no longer cluttered
with images that aren't actually being used outside of the CSS.