`#register` already exists (it's called from `Plugin#name`), so all `*register*` patterns are imho bad.

I think I chose `#require_plugin` as it's the same as in `Vagrant` from where it's called. Plain `#require` would be in align with `register`, but it shadows `Kernel#require`. And feels a bit confusing here.

Dunno, I'm totally fine with `#plugin_required` too.