play(idx) {
      this.load(idx);

      const data = this.playlist[this.index];
      if (!data.howl || !this._media_uri_list[data.id]) {
        this.retrieveMediaUrl(this.index, true);
      } else {
        this.finishLoad(this.index, true);
      }
    }
Play a song in the playlist.
@param  {Number} index Index of the song in the playlist
(leave empty to play the first or current).