Add rename command

[decibel]

Sometimes you decide your original plan for a name was a bad idea. It would be nice if you could rename a change.

To be clear: I am NOT asking that this work for stuff that's already been deployed. Presumably if you wanted that to work you'd need to add some kind of special support to sqitch.plan. That would be nice in the future, but it's a lot more work.

See also request for sqitch remove (#295).

[MightyPixel]

Thumbs up!