add thumb.sh tool

contrib/tools/thumbs.sh

@@ -0,0 +1,23 @@
+#!/usr/bin/env bash
+#
+# shell script for regenerating thumbnails
+#
+
+if [ "$1" == "" ] ; then
+    echo "usage: $0 webroot_dir"
+else
+    cd $1
+    echo "regenerate missing thumbs in $(pwd)"
+    find img/ \
+         -type f \
+         -regextype posix-extended \
+         -iregex '.*\.(png|jpg|gif)$' \
+         -not -execdir test -f '../thm/{}' \; \
+         -exec echo 'generating missing thumb for {}' \; \
+         -exec mogrify \
+         -define jpeg:size=500x500 \
+         -thumbnail '250>x250>' \
+         -path thm/ \
+         -strip \
+         '{}' \;
+fi