• fix bug issue #18 :
  if no snap found, menu generate correctly, and show a warning
• overhaul
• add many new custom variable to /etc/default/grub :
• GRUB_BTRFS_SHOW_TOTAL_SNAPSHOTS_FOUND="true"
  (Show Total of snapshots found during run "grub-mkconfig")
• GRUB_BTRFS_SHOW_SNAPSHOTS_FOUND="true"
  (Show snapshots found during run "grub-mkconfig")
• GRUB_BTRFS_LIMIT="100"
  (Limit the number of snapshots populated in the GRUB menu.)
• GRUB_BTRFS_SUBVOLUME_SORT="descending"
  (Sort the found subvolumes by newest first ("descending") or oldest first ("ascending"). If "ascending" is chosen then the $GRUB_BTRFS_LIMIT oldest subvolumes will populate the menu.)