Add "stutter" capability to the audio file processor.

This adds a new feature to the audio file processor, that I'm calling a "stutter" (for lack of a better term).  When the new option is enabled, it allows the playback of an audio file to resume playing from where it last stopped.

This allows some neat tricks like easy note changes while keeping the playback consistent, or easily changing the volume of a clip without needing a separate automation track.