Unable to parse negative positional argument

Version: v1.6.0

```go
func main() {
	var cli struct {
		Id int `arg:"" help:"Provide an id"`
	}
	_ = kong.Parse(&cli)
	fmt.Printf("cli:%+v\n", cli)
}
```

Unable to parse negative number like: `-100`

```bash
$ go run main.go -100
main: error: unknown flag -1, did you mean "-h"?
exit status 1
```
