Adding -orphans flag to `docker images`

  To see a list of the heads (leaves) of images that are not tagged.
  This is to support the workflow of pruning orphans with a simplified
  example like: docker rmi $(docker images -q -oprphans)

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)