Allow for per-instance extensions

[tsmaeder]

We use msgpackr in multiple places to encode/decode rpc calls. There are a couple of objects that need custom handling before encoding/decoding. We use Extensions to handle those type of objects. However the types of objects that need special handling is distinct per connection. It would be great if we could register Extensions per Packr/Unpackr instance, so that they apply only for a given connection, not everywhere. Feel free to tell me to RTFM if I'm just not understanding how I can do this.

[kriszyp]

This is a very reasonable request, but will need to put some thought into how best to implement.