Get document / supply a document deserializer

[megamidge]

This is a feature request.

It would be handy if either:

• The API synced the ID of each document a long with it's data. Ideally as a non-enumerable property.
  OR
• Allow a deserializer to be passed. Would be a function that is passed the snapshot, and returns the data to put into the store.

May have a look at integrating the first option if I find some time.

[megamidge]

I've addressed the first bullet point #3.

[kazuooooo]

@megamidge
I've released v1.1.0 thank you for your cooperation🥰