Implementation for `d` double type

[dzink]

Thanks for such a powerful library. This is working beautifully to send to Supercollider.

For receiving messages from Supercollider, the lack of a d implementation is throwing this error:

  Invalid argument(s): Unsupported codec typeTag: d
  package:osc/src/convert.dart 40:19   DataCodec.forType.<fn>
  dart:collection                      ListBase.firstWhere
  package:osc/src/convert.dart 38:14   DataCodec.forType
  package:osc/src/convert.dart 330:45  OSCMessageParser.parse.<fn>
  dart:_internal                       MappedIterator.moveNext
  package:osc/src/convert.dart 331:25  OSCMessageParser.parse
  package:osc/src/message.dart 33:31   new OSCMessage.fromBytes
  package:osc/src/io.dart 48:32        OSCSocket.listen.<fn>

I'll see if I can make a run at it if no one else has started this.

[dzink]

See #22