Add helpers for recursively looking up things in ujson.Value

[lihaoyi]

Right now you have to do stuff like

json.objOpt.flatMap(_.get("value")).flatMap(_.objOpt).flatMap(_.get("worker")).nonEmpty)

whereas I'd like to be able to write something like

json.resolve("value", "worker").nonEmpty

[plokhotnyuk]

@lihaoyi What do you think about using of Scala dynamic types for that?

Please see how it is done in the dijon library: https://github.com/jvican/dijon