v0.9.0: Mix sound effects with the soundtrack before ffmpeg encodes.

Sound effects mix with the soundtrack into one track. ffmpeg only encodes
that mix, so the samples do not depend on the ffmpeg version.

Added

• Audio::track builds the soundtrack. Project::audio("music.mp3") still
  works. .volume(), .fade_in(), .fade_out() and .master_fade() shape
  it. .duck_under_voice() lowers it under a clip marked .priority().
• sound("click.wav") starts at the current frame. .play() does not wait.
  Awaiting it waits until the clip ends. .trim() plays a slice of the file,
  which is also how long the wait is.
• The editor draws each effect on the timeline and plays the mix while you
  play or seek. --no-audio leaves effects out. --range keeps only the
  effects inside that part of the video.