Feature request: Realtime updates from Firestore

[edenworky]

It seems that collections and documents don't support the realtime update API in Firestore. Please add support for the onSnapshot function, or otherwise documentation for wrapping the current implementation with observables. My Angular app relies on them.

[soumak77]

Not sure when I'll get to this, but I'll take a PR from anyone that wants to give it a shot.

Here is the official API that needs to be mocked:

• DocumentReference#onSnapshot
• CollectionReference#onSnapshot

onSnapshot should be added as a method to these class

• https://github.com/soumak77/firebase-mock/blob/master/src/firestore-document.js
• https://github.com/soumak77/firebase-mock/blob/master/src/firestore-collection.js