Is there an easy way to toggle the Required attribute of a cli.Flag ?

Now I have to create separate `cli.Flag` flag instances for `Required` and `Optional`, which is quite tedious:


```golang
// HeightFlag ...
var HeightFlag = cli.IntFlag{
	Name:     "height",
	Usage:    "specify height",
	Required: true,
}

// OptionalHeightFlag ...
var OptionalHeightFlag = cli.IntFlag{
	Name:  "height",
	Usage: "specify optional height",
}
```