Add dev script to run rubocop on changed files

script/rubocop-diff.sh

@@ -0,0 +1,23 @@
+#!/bin/bash
+#
+# While you are developing, you can call this script to check all
+# changed files. And then you can also tell Git to check before every
+# commit by adding this line to your `.git/hooks/pre-commit` file:
+#
+#   ./script/rubocop-diff.sh --cached || exit 1
+#
+
+rubocop="`dirname $0`/../bin/rubocop"
+cached="$1" # may be empty
+
+if git diff $cached --diff-filter=ACMR HEAD --quiet; then
+	# nothing changed
+	exit 0
+fi
+
+exec git diff $cached --name-only --relative --diff-filter=ACMR HEAD |\
+	xargs \
+	$rubocop --force-exclusion \
+                 --fail-level A \
+                 --format simple \
+                 --parallel --cache true