Add configurable initial merge strategies

Per my comments in https://github.com/agilgur5/mst-persist/issues/1#issuecomment-508253724, should support something like [`redux-persist`'s State Reconcilers](https://github.com/rt2zz/redux-persist#state-reconciler) as merge strategies. This is basically just to handle state created in `.create(...)`, unless `persist` is called after some actions have been performed (which wouldn't be the standard usage, but is possible 🤷‍♂).

`applySnapshot` is equivalent to `hardSet` but with the model's defaults added. Users should be able to configure if they want a different merge strategy like a shallow merge (`autoMergeLevel1`).