Implement mg_show based on ffplay

[balintlaczko]

While working on #131 I realized that it is very impractical to convert a video to avi just to be able to "quickly" show it. It is okay to say, if you want a motion video or an optical flow video or average image, then you have to convert to avi in order to run those processes, but just to quickly play back a video or show an image, it seems unlikely that users will wait until a video is converted to avi...
FFplay also seems to have more flexible window controls and it is more lightweight than the frame-by-frame while loop with cv2.imshow() (which anyway might not be too accurate on high FPS videos).

[balintlaczko]

This is implemented now as the mode='windowed' form of show(). All videos and images are shown using ffplay as the backend, which seems to be faster, more efficient, and comfortable to use.