Add benchmark rule to Makefile

Change-Id: I5fb5b300750c8ea74b85185fde5c6fb8a96383da
diff --git a/Makefile b/Makefile
index cac4d7a..297bfb8 100644
--- a/Makefile
+++ b/Makefile
@@ -18,3 +18,6 @@
 
 build:
 	go build -v -o ./bin/datok ./cmd/datok.go
+
+benchmark:
+	go test -bench=. -test.benchmem