Feature: allow an exporter to be registered for a base class

Currently JsonMapper.RegisterExporter only registers an exporter for a specific type T. Additional type S derived from T will not use the registered exporter. An "bool inherit" flag passed to RegisterExporter (default false) would be useful.

This could, for example, allow all UnityEngine.Objects to be exported by their name. (Yes, I realize this wouldn't work at import time.)
