allow replication of sublevels (nonbreaking)

I'm not thrilled about the api for this, but I like that it's nonbreaking. I'm trying to allow `level-replicate` to be used to replicate a whole level instance so you can run a cluster of database instances where every node is a master. When a node is spawned, it can contact the other database nodes and start replication of all content (included changes made to sublevels). Is there a better way to do this API?