Allow to set version manually.

I think it'd be great if grunt-bump could specify the version directly, instead of just incrementing one of the current fields. I find myself doing this when I'm working with a test widget that I keep regenerating, and when I regenerate it sets the version back to 0.0.1, but I want to pick back up where I left off.

I'm sure there are other cases where this could come in handy.