Add Vagrant settings feature

Hi,

Great work on Rove, really useful project.

How about this PR for allowing configuration of Vagrant? The thinking is that it allows you to create atomic 'settings' for Vagrant and select them if necessary. I have included the example how I would love to use it at the moment; Port Forwarding but there are other examples that may be useful such as synced folders.

It's a quick attempt but I have tried to follow your existing design but open to any changes/comments?