Add short song detection to music-transfer.py #34

I'd like to be able to configure some song duration which would prevent songs shorter than that length (say, 15 seconds by default) from being transferred, as they're usually not real songs, or they're such short/random songs that they break up the flow of normal music listening.

This will require some way to parse song length from arbitrary audio file formats, possibly via ffmpeg or ffprobe, or by following whatever method is used by the file utility.