Rewrite to support rxdart 0.23.1

[dalewking]

Rxdart 0.23.1 is a major change and eliminates the Observable class.

[dalewking]

Actually it was pretty easy to convert using https://pub.dev/packages/rxdart_codemod, Afterword, I think you only need to:

• import rxdart into RxCommandListener
• make RxCommand extend Stream instead of Observable
• do not call superclass constructor in RxCommand
• Add this method to RxCommand:

  @override
  StreamSubscription<TResult> listen(void onData(TResult event),
        {Function onError, void onDone(), bool cancelOnError}) =>
    _resultsSubject.listen(onData, onError: onError, onDone: onDone, cancelOnError: cancelOnError);

  Haven't tested it yet. I'll see if I can whip up a PR for it.

[escamoteur]

It's on my todo list :-) a PR would be great.

[escamoteur]

Done with new Version 5.0.0