Allow to provide custom data mapping.

[wtrocki]

Sync currently in most of the times returns specific format of dataset:

{ 
  uid: "myid", 
  data: {name:"mydata"}
}

This format is not flexible and in most of the cases it will be better to integrate id into actual data.
Most of the sync implementations doing that anyway.

Example code for formatter:

_.values(syncResponse).map((syncData: any) => {
    // Change format of the data by mapping id's
    syncData.data.uid = syncData.uid
    // Only interested in the actual data from the sync response
    return syncData.data;
  });