support writing values

Introduce support for writing values at location(s) specified by JSON Path. This is an early, perhaps naive implementation, but can be built upon if there is interest. Added some test cases - see there for basic usage.

I am not aware of support for this elsewhere (though I have seen use of Jackson's JsonNode directly, and my own library [adjohnson916/accessor](https://github.com/adjohnson916/accessor) using Spring Expression Language).