Add the Player class to instanciate when calling videojs function

[jsamr]

Description

One can technically inherit from the Player component, but one cannot instantiate it properly, as far as I know.
It would be convenient if the fourth optional argument of the videojs would be a class to instantiate from.
Would be very convenient as an alternative to plugins to change player's behaviour.
I can PR the bellow code if you find it useful.

Snippet

NB : dunno if this snippet works with the Player.extend way of inheriting.

/**
 * Doubles as the main function for users to create a player instance and also
 * the main library object.
 * The `videojs` function can be used to initialize or retrieve a player.
 * ```js
 *     var myPlayer = videojs('my_video_id');
 * ```
 *
 * @param  {String|Element} id      Video element or video element ID
 * @param  {Object=} options        Optional options object for config/settings
 * @param  {Function=} ready        Optional ready callback
 * @param  {Function=} PlayerClass  Optional, the constructor of a class inheriting from Player                  
 * @return {Player}                 A player instance
 * @mixes videojs
 * @method videojs
 */
let videojs = function(id, options, ready, PlayerClass=Player) {
    if( ! PlayerClass.prototype instanceof Player && ! PlayerClass === Player ) {
         throw new TypeError('PlayerClass argument must the constructor of a class inheriting from Player, or the Player class itself.');
    }
    // ... 
    // Element may have a player attr referring to an already created player instance.
    // If not, set up a new player and return the instance.
    return tag['player'] || Player.players[tag.playerId] || new PlayerClass(tag, options, ready);

}

[gkatsev]

If you already have a player, why not just create a new instance of it yourself? The videojs function is basically so that you don't need to create the player manually.

[gkatsev]

There's a new PR for this: #3458

@adam187 @sveinburne can you elaborate on the use-case? Why is it necessary over just having you create the Player manually yourself, since you already have a reference to it?

var player = new MyPlayer(playerEl, {});

[adam187]

@gkatsev This use case looks different than mine. I will describe my use case in depth in PR.