Duplicate flag check and Parse order modification request

[fcharlie]

I use kong to parse command line, but I found some problems while using it, one of them is "duplicate flag".

1. the presence of the same negatable flag in different subcommands will cause panic.

package main

import "github.com/alecthomas/kong"

var CLI struct {
	Rm struct {
		Force     bool `help:"Force removal."`
		Recursive bool `help:"Recursively remove files."`

		Paths []string `arg:"" name:"path" help:"Paths to remove." type:"path"`
		Fast  bool     `name:"fast" negatable:"" help:"fast check"`
	} `cmd:"" help:"Remove files."`

	Ls struct {
		Paths []string `arg:"" optional:"" name:"path" help:"Paths to list." type:"path"`
		Fast  bool     `name:"fast" negatable:"" help:"fast check"`
	} `cmd:"" help:"List paths."`
}

func main() {
	ctx := kong.Parse(&CLI)
	switch ctx.Command() {
	case "rm <path>":
	case "ls":
	default:
		panic(ctx.Command())
	}
}

2. subcommands cannot override command flags:

package main

import "github.com/alecthomas/kong"

var CLI struct {
	Config []string `help:"config value" short:"c"`
	Rm     struct {
		Force     bool `help:"Force removal."`
		Recursive bool `help:"Recursively remove files."`

		Paths []string `arg:"" name:"path" help:"Paths to remove." type:"path"`
	} `cmd:"" help:"Remove files."`

	Branch struct {
		Paths  []string `arg:"" optional:"" name:"path" help:"Paths to list." type:"path"`
		Create bool     `name:"create" short:"c" help:"Create branch"`
	} `cmd:"" help:"List paths."`
}

func main() {
	ctx := kong.Parse(&CLI)
	switch ctx.Command() {
	case "rm <path>":
	case "ls":
	default:
		panic(ctx.Command())
	}
}

In addition, it would be great if commands like git -c X=value switch -c new-branch could be supported. （First -c set config to command）

[alecthomas]

Thanks for the bug report!