static extend (name, player) {

    if (typeof this.players_ === 'undefined')
      this.players_ = {};

    if (typeof this.players_[name] !== 'undefined')
      console.warn(`Player with name ${name} already exists`);

    if (typeof player.play_ === 'undefined')
      console.warn('Player does not contain a play_() function');

    this.players_[name] = player;
  }
Extends FLIP to be able to play. Allows for the use of libraries when it
comes to playback. The functions in the player object are all copied and
bound to the instance of the FLIP Helper on instantiation.

@param {String} name - The name of the player, e.g. <code>'rAF'</code>.
@param {Object} player - The object with the playback functions.
@param {Function} player.play_ - The playback function to use.