build: Add compare script (#426)

Simple shell script to help check the differences between debug and
non-debug versions, though can also be used to compare cross-browser
differences too.

scripts/compare.sh

@@ -0,0 +1,39 @@
+#!/bin/sh
+check_browser() {
+	if [ "$1" != "firefox" ] && [ "$1" != "chrome" ] && [ "$1" != "opera" ] && [ "$1" != "edge" ]; then
+		echo "'$1' is not a valid browser"
+		exit 42
+	fi
+}
+
+compare() {
+	echo "compare $1 with $2"
+	git diff --no-index "build/script-cache/$1" "build/script-cache/$2"
+}
+
+echo "Usage:"
+echo "    <browser> <browser>"
+echo "    <browser> <browser> d"
+echo "    <browser> d"
+echo
+echo "You may want to run the following first"
+echo "    node scripts/build.js --browser all --debug"
+echo "    node scripts/build.js --browser all"
+
+if [ ! "$1" ] || [ ! "$2" ]; then
+	echo "Missing first or second arg"
+	exit 42
+fi
+
+if [ "$2" = 'd' ]; then
+	check_browser "$1"
+	compare "$1" "$1-debug"
+else
+	check_browser "$1"
+	check_browser "$2"
+	if [ "$3" = 'd' ]; then
+		compare "$1-debug" "$2-debug"
+	else
+		compare "$1" "$2"
+	fi
+fi