feat: Allow kong.Path to describe remaining unparsed args

[boblail]

As Kong traces a sequence of command line arguments, it parses them and appends them to the parsed Path sequence. For each element in Path, these is a corresponding sequence of unparsed arguments. This change enables Path to yield these.

I have a package that uses Kong's hooks to instrument Kong applications (to monitor usage, reliability, etc of internal tools). I would like to instrument the commandline arguments as well.

This change would enable it to work roughly as follows:

func (Foo) BeforeApply(app *kong.Kong, ctx *kong.Context, t *Tracker) error {
	command := []string{ctx.Model.Name}
	var args []string

	for _, path := range ctx.Path {
		if path.Command != nil {
			command = append(command, path.Command.Name)
			args = path.Rest()
		}
	}

	app.Exit = t.exit(app.Exit)

	t.WithCommand(strings.Join(command, " ")).WithArgs(args)
	return nil
}

[alecthomas]

Seems reasonable, but could you add a test? And I think a better name would be Remainder()