Add features from “Regenerate Thumbnails” plugin

#1 would make sense to me, though it’s not really what this plugin is for. FRT intentionally removes all thumbs, in case the existing ones are incorrect, or you changed the quality, or anything else. But still, not a bad idea, so we’ll look into it further. I would note, that last I checked, even if we don’t delete existing thumbs, WP still regenerates all the registered sizes, so implementation is a little more complicated than just “don’t delete registered thumbs”.

#2 is already what FRT is supposed to do. As I said before, it deletes all thumbs prior to generating new ones.

#3 already exists in WP-CLI, and I’m not certain what it would benefit for FRT to add our own WP-CLI extension. We’ve already made it so that FRT is resumable and pretty fault-tolerant, though I’m sure there’s some way it can break, whether it’s having 50 registered sizes or a really low PHP timeout, or anything along those lines.