Implement stringref extension

[BurtHarris]

The http://cbor.schmorp.de/stringref CBOR extension (tags 25 and 256) permit substantial compression of CBOR encoded objects with repetitive string values. It seems adding this to the CBOR encoder and decoder would introduce relatively little overhead in cases where it isn't used, and could actually run faster when decoding objects where the compression is effective.

This is a topic that interests me, and I'd be willing develop a pull request for it if you are interested.