Support other well known types. e.g. StringValue, Int64Value

These protos are useful for representing optional values where you want to disambiguate empty from nil.

I'm happy to work on a PR. The question is what expected behavior should be.

Given:

```protobuf
message M {
  StringValue str = 1;
}
```

JSON serializes to:

```json
{}
{"str": ""}
{"str": "some value"}
```

The first two probably shouldn't be equivalent.