Allow customisation of the control widgets

It would be nice if the `Chewie` user could remove / add / reorder the control widgets. For example users would want to remove the ability to fullscreen or mute. Or they might want to add a button to switch between different versions / encodings of a video.

Instead of adding `allowFullScreen`, `allowMuting`: What about a list of widgets for the materialBottomControlBar? And one for cupertinoBottomControlBar plus cupertinoTopLeftControl and cupertinoTopRightControl? The `ChewieController` (https://github.com/brianegan/chewie/pull/99) would make it much easier for the user to add own or the original widgets.