m3u, folder-structure and smaller Docker image

• New options
  • --m3u: Create m3u playlist files for each downloaded playlist (audio only)
  • --folder-structure=server Stores musik files in the same folder structure as on server (audio only)
  • --folder-structure=list creates subdirs for each list (audio & photo)
• Changed option: --flat replaced by --folder-structure=flat
• Improved error handling: In case of error, skips files and continues with others.
  Instead, error statistics are returned on exit.
• Docker container no longer runs as root user & and image is much smaller

Migration from 2.x:

• Change -f to -f flat or --flat to --folder-structure=flat
• Always check error statistics for failed files or lists