Docdiff: configure deletions/additions

[humitos]

Just a note here for now, but I could see this implementation drifting closer to either a multiposition slider, pair of toggle bars, or even just a dropdown instead (I lean towards this). There are a few modes that I'm finding useful, depending on the pull request contents:

On, show deletes and additions
On, only show additions
Off

This doesn't need to affect the direction here yet, but will be something we want to consider in the future. Just noting so we don't cement too much of our implementation around a single, binary toggle bar. For the purpose of turning the diff off, this likely works fine for now.

(from #63 (comment))