Makefile: Add fmt target to run go fmt over all non-vendor code

Borrowed from libnetwork's Makefile

Signed-off-by: Euan Harris <[email protected]>

Makefile

@@ -67,11 +67,16 @@ lint:
 	@echo " -> $@"
 	@golint -set_exit_status ${PACKAGES}
 
+fmt:
+	@echo " -> $@"
+	@test -z "$$(gofmt -s -l . | grep -v vendor/ | grep -v ".pb.go$$" | tee /dev/stderr)" || \
+                (echo "👹 please format Go code with 'gofmt -s -w'" && false)
+
 cyclo:
 	@echo " -> $@"
 	@gocyclo -over 20 ${CYCLO_PACKAGES}
 
-check: vet lint
+check: vet lint fmt
 
 test:
 	@go test -short -v -cover $(TEST_ARGS) ${PACKAGES}