Add support for shallow diff

This file includes support for shallow diff. I'm not sure we should include in this repo, but we needed for a project and it might be useful in the future. I wrote a couple basic tests and one edge case I ran into.
[JSONAdditions with Diff.fmp12.zip](https://github.com/geistinteractive/fm-json-additions/files/5698283/JSONAdditions.with.Diff.fmp12.zip)
