Add LaTeX support (I mean inline LaTeX, not this thing).

EDIT: Based on comments below, I think I misused the word "inline". What I meant was simply the ability to type LaTeX directly into comments and posts. How it gets rendered doesn't matter much to me; some legitimate objections have been raised, but I don't feel like hard math gets used enough on the site that this would get out of hand. Restricting its use to posts rather than comments might be a good compromise.