Be more helpful in case of wrong usage

dotfiles/gittum/bin/sibling

@@ -66,6 +66,7 @@ while true; do case $1 in
     -r|--remote-name)   remote_name=$2; shift 2 ;;
     -s|--sibling)       sibling=$2;     shift 2 ;;
     -v|--verbose)       verbose=true;   shift   ;;
+    -*)                                 usage   ;;
     "")                                 break   ;;
     *)                  project=$1;     shift   ;;
 esac done