Report amount of decoded data

[anonghuser]

The library should be able to report what portion of the provided buffer has been decoded, so that the caller can know if there is extra data after the messagepack structure and where it starts.

[anonghuser]

FWIW I am able to hack a solution with

    const options = {codec: msgpack.createCodec({})};
    options.codec.decode = (old => function(decoder){
      options.decoder = decoder;
      return old.apply(this, arguments);
    })(options.codec.decode);

    msgpack.decode(buf, options);

    console.log('decoded amount:', options.decoder.offset);

But something cleaner and documented would be welcome.