feat(firestore): Add SnapshotOptions

[davideast]

Mostly addresses #58.

This PR adds support for SnapshotOptions within the data mapping functions like docData() and collectionData(). The existing options object only supported idField. This PR merges that configuration with SnapshotOptions to avoid a breaking change. We will address the addition of SnapshotOptions for non data mapping functions in the next release as it would require an awkward API or a breaking change.

collectionData(colRef, {idField: 'userId', serverTimestamps: 'estimate'});
docData(docRef, {idField: 'userId', serverTimestamps: 'estimate'});