Views: Add "Limit terms by vocabulary" for terms

This probably won't be used by too many people but it was useful for me.

By default in regcode you can only have 1 vocabulary attached to a regcode, but for my purposes I wanted multiple to be attached. I was able to programmatically add the tags to the regcode just fine.

However I wasn't able to disable it to on the views page because the field handler was limiting it only to the regcode vocab (Which is saving it as a vid instead of the machine name which messes up strongarm, but that's another issue entirely).

Attached is a patch that allows you to select if you want to limit the field/term handler by a vocab.