ENH: Allow videos to be iterable.

[masasin]

There's a function to iterate over all the frames in a video, but it would be much prettier to just for frame in video: directly, especially if you're already controlling what you're getting using the context manager.

[anbergem]

Great proposal! My first thought is that this would have to be done by extending the cv2.VideoCapture class, adding an __iter__ method. Any inputs on this?

I'm currently developing tests, but I'll be sure to add this soon!