(rules.json) Add schema for comments
(FirebaseCollection) Replace metaRef option with metaFunction

This allows us to be much more flexible when grabbing other relevant data
around the Firebase.

Instead of relying on one particular use-case with metaRef, we implement a
metaFunction option. The metaFunction gets passed the first snapshot from the
collectionUrlOrRef. It can then do any post-processing, such as accessing data
from another Firebase reference. Finally, it MUST call doAdd(), which must be
passed a Firebase reference as its first parameter, and an object with any
additional data (that we might have accessed from another Firebase reference,
for example) as its optional second parameter.

The reference and extra data passed into doAdd() will then be combined to form
the final collection item. Awesome!