Write out ruler metadata state when serializing ruler objects

[tom-osika]

As of #121, when serializing rulers, the ids of each ruler are written out to json. When loading the json and constructing the rulers again, the ids are used to re-derive the state of the metadata generators, so that the user can pick up where they left off with annotating. This approach could be improved upon by serializing the metadata generators themselves, instead of re-deriving their state using just the ids.