fix: When a Grammar combines flags with passthrough args, see if an unrecognized flag may be treated as a positional argument

[boblail]

Issue

Given a grammar like this:

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.

Proposal

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"}.

[alecthomas]

Thanks!