load(idx) {
      let index = typeof idx === 'number' ? idx : this.index;
      if (index < 0) return;
      if (!this.playlist[index]) {
        index = 0;
      }
      // stop when load new track to avoid multiple songs play in same time
      if (index !== this.index) {
        Howler.unload();
      }
      this.index = index;

      this.sendLoadEvent();
    }
Load a song from the playlist.
@param  {Number} index Index of the song in the playlist
(leave empty to load the first or current).