Bugfix for FPS, running live

[cstenkamp]

• fix frame_count.start being accessed when it's not set (sorry, was wrongly named in the commitmessage)
• fix wrong FPS-count for some videos by using ffprobe instead of cv2's mechanism (I think it doesn't work correctly for .avi's)
• add kwarg to create a video purely from the poses without the original video underneath it
• add possibility to run live, without saving (such that it becomes more similar to tf_pose_estimation's run_video at https://github.com/ildoonet/tf-pose-estimation/blob/master/run_video.py)