Allow to manage multiple stashes in a single backend

[nir0s]

As it is now, ghost can only manage a single stash per storage backend. This is due to the fact that it doesn't allow to declare a "path" for the stash. This also means that only a single passphrase can be applied per backend.

By allowing multiple "paths" (i.e. path in vault, index, in ES, table in tinydb/sqlalchemy, etc..), multiple stashes will be accessible.