TransitionImage class and fetchImage function added

transition_image.dart

fetchAndUpdateImage([String url = 'https://picsum.photos/300/200']) async {
  final response = await http.get(url);
  floop['image'] = TransitionImage(Image.memory(response.bodyBytes));
}

class TransitionImage extends FloopWidget {
  final Image image;
  const TransitionImage(this.image);

  @override
  Widget build(BuildContext context) {
    return Opacity(opacity: transition(1500), child: image);
  }
}