Add empty subcommand

I'm not sure whether or not this command should be added to the `config` plugin.

I picked the most clearly-not-a-synonym-for-`unset` name I could (short of something outrageous like `nuke`) and I added a safety check so that it will fail in the event that somebody *does* mistake it for `unset`.

With all these considerations in mind, it still feels like adding a massive footgun for an uncommon task that could be accomplished with `plushu config $app | sed 's/=.*//g' | xargs plushu config:unset $app`.