morebits: Ignore image/category embeds while removing links

Fixes #231. @atlight Could I get a second pair of eyes on that regex? I'm pretty sure it's correct but I might have made a simple mistake.
I didn't test it (no time right now...), but it seems that JavaScript doesn't support lookbehinds. In any case ?! is a lookahead, not a lookbehind. Could you test this anyway in a browser and see what happens?