cmd/mbf: explicit help command

Not having this command is counterintuitive.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/mbf/main.go

@@ -241,6 +241,12 @@ func main() {
 			)
 	}
 
+	c.Command(
+		"help",
+		"Show this help message",
+		func([]string) error { c.PrintHelp(); return nil },
+	)
+
 	c.MustParse(os.Args[1:], func(err error) {
 		if cache != nil {
 			cache.Close()