Leverage guest capabilities

I originally suggested that this involve actions, but actions are provider-specific, so this is more for guest capabilities.

The idea is that this would be a first step toward making this plugin usable by third-party providers like vagrant-digitalocean and vagrant-rackspace. If it's implemented via an action, then those providers can include vagrant-rsync as a dep and all sync folders via the same code path.

Ref: https://github.com/mitchellh/vagrant-rackspace/pull/15