Validate value via pointer

[jeromegn]

In our use case, we're accepting small patches to a JSON document and we'd like to be able to validate the changed path only.

From a public API standpoint, we'd have our schema(s) and would be able to pass a pointer to a function to validate only the valued changed at the pointer.

Looking at the internal API structure, I don't think this would be too hard. If this is something that might benefit others, then I could make a PR.

I'd like some feedback on the idea about its usefulness and feasibility!