Highlight TODO/FIXME etc. in pull request diffs (BB-7536)

I know ReviewBoard does this (or did in the install that I used), but it was a nice feature: TODO or FIXME in the text of a diff was highlighted so it stuck out. I'd love to see syntax highlighting too, but this sort of thing would be an awesome start, so that when you're skimming a pull request, things like that jump out.