Support for custom video player controller

[matwright]

Small update which allows you to pass a custom VideoPlayerController to the component. For example, this is very useful if you want to stream otherwise unsupported video types such as M3U8

Simply pass the VideoPlayerController through to the factory as such:

  StoryItem.pageVideo(
      ..., 
      controller: controller,
      playerController  VideoPlayerController.network(some_uri_for_a_network_m3u8_file)
    )

the playerController field is entirely optional so there is no backward breaking issues.

[matwright]

I decided that the custom PlayerController probably isn't a suitable approach for the use case described.