Add sync or mirror functionality to data store

[coreybutler]

From ngn-core created by gbdrummer : ngnjs/ngn-core#1

We currently don't have an automated way to relay events from one store to another. For example, if I am storing different pieces of data about a particular UI element in multiple different stores, we now have to manually track down the data in each store and update it.

It would be nice to have a way to link different bits of data in different stores.