Custom buttons

I believe there's room for providing custom buttons to the player, and associate those with external handlers.

In my application I've added an HD button that appears next to the full-screen button and it worked great, see - https://github.com/fluttercommunity/chewie/compare/master...loudinthecloud:flutter_chewie_with_qualities:add_hd_button

Not sure if this is the best design, but it gets the job done.

I think the user should be able to provide a list of buttons in the chewie configuration, and perform any action he needs externally. I think the most common features will be to implement a stateful button, with handlers for each state. Please let me know if you'd like a pull request, and what would be the requirements.