add config to let user choose whether to render number64 type such as int64, uint64 as integers or string

[anotherGoogleFan]

In some scenarios, we need to render number64 type such as int64, uint64 as integers instead of string values, but found we cannot. So I add an option in config to fix this. And this will also resolve https://github.com/gogo/protobuf/issues/718