Track current stream offset? #211

In some applications, namely indexing of msgpack streams, it would be very useful to tell the total offset (not the buffer offset) of the last object returned by the Unpacker iterator. In code:

for packet in data:
    unpacker = Unpacker(...)
    unpacker.feed(data)
    for object in unpacker:
        print(unpacker.offset)

Would print for example 0, 1234, 4567, and so on.