Darken & embolden GFM Headings

Make headings in GFM documents stand out by darkening and emboldening them.

Do you even want syntax-specific markup in zen, and would this be the right place for it? I'd personally love to see Zen serve as a beautiful markdown editing view, so I'm also wondering if there is a way to force soft-wrap, etc.

Note: While I personally like this style w/ the dark theme, I haven't tried it out on the light theme yet.