Video plugin conveniences

[talmo]

There are a lot of differences between OpenCV, FFMPEG and av backends on different platforms and environments. It'd be great to have more convenient control over these and how they're handled in Video and VideoBackends.

API:

• sio.set_default_video_plugin(): Global preference to specify default video plugin for all subsequently loaded videos.
• Labels.set_video_plugin(): Reopen all media videos with the specified plugin.
• Video.set_video_plugin(): Set the specified plugin and reopen the video.
• Video.open(..., plugin=...): Add plugin kwarg to existing open function.

It would also be nice to add the media video plugin names to Video and sio.load_video docstrings. It would be nicer if they were a bit more flexible to accept aliases (e.g., "cv", "cv2", "opencv", "ocv", case-insensitivity, etc.).