Add dependency of `go fmt` in `Makefile` (#553)

This fix adds the dependency of `go fmt` in `Makefile`
so that any new check in will have to properly fmt the source code
with `gofmt -s -w`.

Makefile

@@ -20,7 +20,7 @@ docker: deps
 	docker tag $(DOCKER_IMAGE_NAME):latest $(DOCKER_IMAGE_NAME):$(DOCKER_VERSION)
 
 .PHONY: deps
-deps:
+deps: fmt
 	go get ${BUILD_VERBOSE}
 
 .PHONY: test
@@ -55,6 +55,12 @@ core/zmiddleware.go core/dnsserver/zdirectives.go: middleware.cfg
 gen:
 	go generate coredns.go
 
+.PHONY: fmt
+fmt:
+	## run go fmt
+	@test -z "$$(gofmt -s -l . | grep -v vendor/ | tee /dev/stderr)" || \
+		(echo "please format Go code with 'gofmt -s -w'" && false)
+
 .PHONY: distclean
 distclean: clean
 	# Clean all dependencies and build artifacts