Adding support for dismissible jokes

dismissible.dart

_refreshAction() {
  setState(() {
    _response = http.read(dadJokeApi, headers: httpHeaders);
  });
}

...
  
if (decoded['status'] == 200) {
  _displayedJoke = decoded['joke'];
  return new Padding(
      padding: const EdgeInsets.all(16.0),
      child: new Dismissible(
        key: new Key("joke"),
        direction: DismissDirection.horizontal,
        onDismissed: (direction) {
          _refreshAction();
        },
        child: new Text(_displayedJoke, style: jokeTextStyle),
      ));