v0.20.0

• Add new parameter gain_during_overlay to pydub.AudioSegment.overlay which allows users to adjust the volume of the target AudioSegment during the portion of the segment which is overlaid with the additional AudioSegment.
• pydub.playback.play() No longer displays the (very verbose) playback "banner" when using ffplay
• Fix a confusing error message when using invalid crossfade durations (issue #193)