Allow setting arbitrarily nested (inside hashes) values in a Change

[alechenninger]

...in such a way that ignores the rest of the hash and only changes or adds the key in that hash.

[alechenninger]

Can use JsonPath I think: https://github.com/jayway/JsonPath

Supporting definite paths would be simple I think. Indefinite paths not so much, because the actual paths that match are a function of the source (and so different for each source in hierarchy). I think we'd have to match every path in target and descendents and then put all of those at target, removing from descendant (taking into account whether key is a merge key or not).