…that isn't passthrough

…nrecognized flag may be treated as a positional argument

Given a grammar like this:
```golang
var cli struct {
	Args []string `arg:"" optional:"" passthrough:""`
}
```

The first positional argument implies that it was preceded by `--`, so subsequent flags are not parsed.

If Kong parses `cli 1 --unknown 3`, it will populate `Args` with `[]string{"1", "--unknown", "3"}`.
However, if Kong parses `cli --unknown 2 3`, it will fail saying that `--unknown` is an unrecognized flag.

This commit changes the parser so that if an unknown flag _could_ be treated as the first passthrough argument, it is.

After this change, if Kong parses `cli --unknown 2 3`, it will populate `Args` with `[]string{"--unknown", "2", "3"}`.