Fixed cover art being dropped on file split

[leoncvlt]

Noticed that even though my source file had an embedded cover art, it was being lost during the split. This fix involves:

• Removing the -an flag from the split command (as the cover art is embedded in a video stream, we need the video now)
• Moving the -ss -t flags before the -i (See https://superuser.com/questions/758338/keep-album-art-with-ffmpeg-while-cutting-a-mp3-file)

Because of how the ffmpy library syntax works, that wasn't possible by keeping a single source file, so I had to split the split (heh) process into one ffmpeg command for each track - it's a tad bit slower, but can't think of another way to do this.