feature: implement stringref extension

[leventov]

Is your feature request related to a problem?
Improve compression when there are a lot of repeated tags.

Describe the solution you'd like
http://cbor.schmorp.de/stringref

[a-hennig]

about a year ago I also though about adding this ... in my case compression factor would be huge.
I couldnt find a way to add this in own tags, because I need the context of the current encoders/decoders ... something that would lead me to the currently valid lookup map. And using a singleton would just kill concurrency / performance.

any chance to enhance UnmarshalCBOR([]byte) to UnmarshalCBOR(context, []byte) ??