Feature: Offline mode

[AndersDJohnson]

Something like https://github.com/yeldarby/offline-firebase, caching to localStorage. Might need configurability & some mechanism (e.g. LIFO) to manage size so we don't exceed browser data quota.

[AndersDJohnson]

Might also want some way of marking certain paths as locally "dirtyable" where they temporarily ignore any updates from the server until we explicitly instruct to abandon or push changes. See #39.

[AndersDJohnson]

This may be better as an external library, with an interface used by this module, which could also support the current in-memory mode as another implementation of that interface.