Allow for finer-grained permissions

This adds `blockhat.unhat` and `blockhat.return`, makes some messages clearer an
d fixes a couple of spelling mistakes.

An example use case for the new permissions is if you want your players to be ab
le to have whatever hat they want, but you don't want them to use their inventor
y. If you only give them `blockhat.hat.items` they can't use `/unhat`, hence why I added `blockhat.unhat`. However, with `/hat <id>` there is the potential for
players to abuse it and spawn lots of items in, so the `blockhat.return` permission means that the block is restricted to being a hat (assuming that there is also a plugin installed to prevent players from taking it out of the helmet slot).