Process all image/link URLs during github rewrites

[revin]

The github URL-rewriting markdown-it plugin was accidentally only doing a single URL replacement per html_block; this change recursively processes every occurrence instead.

Fixes #415.

[bcoe]

@revin 👋 I haven't had a chance to dig into this yet, but I'd be interested to see if we can replace the regex with //g and avoid the recursive step -- I bet we can figure something out.

[bcoe]

as an alternative, if we can't, we could switch the recursive step to a stack structure and at least eliminate the chance of a stack overflow.