Add fmt check command

magefile.go

@@ -216,8 +216,22 @@ func Fmt() {
 }
 
 // Checks if the code is properly formatted with go fmt
-func FmtCheck() {
+func FmtCheck() error {
+	args := append([]string{"-s", "-d"}, GoFiles...)
+	c := exec.Command("gofmt", args...)
+	out, err := c.Output()
+	if err != nil {
+		return err
+	}
 
+	if len(out) > 0 {
+		fmt.Println("Code is not properly gofmt'ed.")
+		fmt.Println("Please run 'mage fmt' and commit the result:")
+		fmt.Print(string(out))
+		os.Exit(1)
+	}
+
+	return nil
 }
 
 func Lint() {