rename relrenderer

This was sorta a weird extension I wrote to add link relations, I'm not really
using it anywhere, plus it was doing that AND building the outline, so
now it's just outline renderer.
.markdown -> .md

This also involved moving a really terrible codepath that tried to load
.markdown, and if that failed, loaded .md instead. I never should have
done that, but I was lazy...
small style change around rescue.

I thought that I wanted to pull this exception up, but it's actually pretty okay.
remove one last reference to .markdown