Evaluate pushing [u]int/float64 to output plugins

[remingtonc]

Per #6 we replicate/customize (was originally a patch) jsonpb which emits uint64 values as strings to instead make them uint64s. It appears this was largely done to maintain correctness with JavaScript alongside some JSON specification language so this is a fine workaround, however if we can easily push that deserialization functionality to the outputs it would probably be a better solution and explicitly acknowledge the intended functionality of jsonpb.