[Feature Request] Provide a way to customize cross tab observer implementation

[momocow]

Storage events are only able to detect changes of localStorage and sessionStorage.

Since I'm working on a driver for value storage of userscripts (e.g. Tampermonkey), I need a way to customize the cross tab observer using GM_addValueChangeListener in order to make my driver compatible with localForage-observable.

Thanks for a nice package :)