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`.

## The case against

As `empty` is more closely associated with the adjective "empty" than the verb "empty", this could be misconstrued as a condition test command ("is the config empty?"). Since `plushu` in general doesn't *have* any commands like that (and you could obviously test that with `[[ -z $(plushu config $app)]]`), it would be weird for someone to think that, but it's a plausible mistake that a curious cat could stumble into (once).

Ambiguity aside, it's a footgun for an uncommon task that could otherwise be accomplished with the one-liner:

```bash
plushu config $app | sed 's/=.*//g' | xargs plushu config:unset $app
```

## The case for

The rationale being that the one-liner is a complex command requiring two network round-trips and a `sed` pipe step (not including all the server churn), for something that can be blindly accomplished with *one character* on the server.