0.17.0

Added

• Gapless multi-track playback — crossing from one file to the next in a
  multi-track (mp3-folder) book no longer has a brief stall while the next
  file loads. A second, hidden <audio> element quietly preloads the
  upcoming track well ahead of the boundary; the moment the current one ends,
  playback hands straight off to the already-buffered element instead of
  starting a fresh load. Manual chapter jumps and seek-bar scrubbing are
  unaffected — they still load on demand, same as before.