Make `wp plugin scaffold` work

I just caught myself mindlessly typing `wp plugin scaffold` and being surprised that it didn't work. That's a textbook example of the [principle of least astonishment](http://en.wikipedia.org/wiki/Principle_of_least_astonishment) being broken.

We should alias it to `wp scaffold plugin`.

If we ever add a `wp child-theme` top-level command, we should also alias `wp child-theme scaffold` to `wp scaffold child-theme`.