Restructured the generate command

RE: my basecamp post about changing the generate command structure

Had a bit of spare time this morning, so I moved some things around to enable separating out the `generate` command and the generators. We've gone from `diorama generateModel` to `diorama generate model` etc. I think this makes it a bit easier to read the help text, too.

Not entirely sure the refactor is perfect in some parts, but the idea is there.