[script] Add simple benchmark script

`benchmark` can be invoked with an executable file after it. It will
output the running time in milliseconds. Depends on gnu date being
installed on the system.

scripts/.local/bin/benchmark

@@ -0,0 +1,18 @@
+#!/usr/bin/env sh
+# Benchmarking script using gnu date
+# can be used to output running time of script in milliseconds
+
+# usage: benchmark scripttorun
+#
+# Note: script itself uses roughly 1 millisecond (on my system), so subtract that
+# from https://unix.stackexchange.com/a/334152
+
+START=$(date +%s.%N)
+# do something #######################
+
+"$@" 1>/dev/null 2>/dev/null
+
+#######################################
+END=$(date +%s.%N)
+DIFF=$(echo "scale=3; (${END} - ${START})*1000/1" | bc)
+echo "${DIFF}"