I want to use key code instead of string.

I would like to add a function:

```
func KeyCode2String(key int) (k string, err error) {
	for keyStr, keyCode := range keyNames {
		if keyCode == C.int(key) {
			return keyStr, nil
		}
	}
	return "", errors.New("Invalid key specified.")
}

key, _ = KeyCode2String(112)  // F1
robotgo.KeyToggle(key)
```

But I can't use keyNames.
