function bindProgress() {
    _config2.default.audio.removeEventListener("progress", _progress2.default.handle);
    _config2.default.audio.addEventListener("progress", _progress2.default.handle);
  }
As the audio is loaded, the progress event gets fired. We bind into this
to grab the buffered percentage of the song. We can then add more elements
to show the buffered amount.

@access private