Provide --debug option for very verbose execution

This effectively executes grml-debootstrap under 'set -x'
(including the chroot script), so it's easy to generate an
execution log via e.g.:

  grml-debootstrap ... --debug --force 2>&1 | tee -a grml-deboootstrap.log

and providing it to grml-debootstrap developers for further
investigation.