Add data transforms on outbound data

[jimmiebtlr]

First, thanks for a very clean and well written repo, and sorry for the torrent of issues and prs.

Not sure if there is a clean way to implement this. I'd like to be able to transform data before sending to storage.

Ex.

A field that has some placeholder when a value was missing could be tossed out if not of a number type

{"someField": "No value"}  => {}

and

{"someField": 123} => {"someField": 123}

Alternatively, maybe something with being able to toss out a field when it doesn't match the schema?