[Improvement] Disable flags suggestion after -- argument

[sagan]

Hi

Cobra is fully POSIX-compliant, in which the argument -- terminates all options; any following arguments are treated as non-option arguments, even if they begin with a hyphen. (See here for more)

Therefore, the flags suggestions should be disabled if user has already typed a pure "--" token, plus any space, before cursor.

I made a small change to func findSuggestions in cobra-prompt.go and tested it should work

func findSuggestions(co *CobraPrompt, d *prompt.Document) []prompt.Suggest {
	command := co.RootCmd
	args := strings.Fields(d.CurrentLineBeforeCursor()) // changed from using d.CurrentLine()

	// ... omitted

	// replace this 2 lines
	// command.LocalFlags().VisitAll(addFlags)
	// command.InheritedFlags().VisitAll(addFlags)
	flagsTerminated := false
	args2 := args
	if d.GetWordBeforeCursor() == "--" {
		args2 = args[:len(args)-1]
	}
	for _, arg := range args2 {
		if arg == "--" {
			flagsTerminated = true
			break
		}
	}
	if !flagsTerminated {
		command.LocalFlags().VisitAll(addFlags)
		command.InheritedFlags().VisitAll(addFlags)
	}
}