Implement Bag Serialization

I'd be interested in seeing `bag.serialize(to: , format: )` or something similar implemented so that  bags could automatically serialized to zip or tar..gz. This is documented in https://tools.ietf.org/html/draft-kunze-bagit-06#section-4 

Is this already on your roadmap? I'd probably be up for submitting a pull request for it if it isn't.