StringSlice CLI should replace default values

[edoardottt]

• Fix StringSlice CLI should replace default values #95
• Add tests for StringSlice CLI should replace default values #95
• Fix Default Port Input (Same as StringSlice CLI should replace default values #95)
• Add tests for Default Port Input
• Minor changes (expected and value in tests reversed)

I can confirm PortVarP() is affected as well:

[...]
func main() {
	testOptions := &Options{}
	flagSet := goflags.NewFlagSet()
	flagSet.CreateGroup("info", "Info",
		flagSet.PortVarP(&testOptions.Port, "port", "p", []string{"1"}, "port test"),
	)
	if err := flagSet.Parse(); err != nil {
		log.Fatal(err)
	}
	fmt.Println(testOptions.Port)
}

go run port-flag.go -p 2
(1,2)

Honestly I don't like this solution, it breaks the concept of having the StringSlice exactly equal as []string (And so reflect.Type).
That said, personal considerations:

• This solution is backward-compatible (in my opinion the most desired property).

• (flagSet *FlagSet) Parse() calls flagSet.CommandLine.Parse(os.Args[1:]) (ref: https://github.com/projectdiscovery/goflags/blob/main/goflags.go#L96) and it's not possible to override the second method. In that method the way used to set a flag value is to call Set() and (as far as I know) there is no way to distinguish between user-inputted values and default ones. So there is no way to override the default value out of using the Set() method.

• Each input flag should have the notion of default value, that's why I've thought to change StringSlice to a struct holding the Default indicator.

• Maybe another solution could be also to change optionMap to be accessible from outside (OptionMap) and insert the boolean value in that struct.

TBH I don't have a clear abstract view of the code. I'm open to discuss, get suggestions and change completely the solution.

This PR closes #95.

[Mzack9999]

@edoardottt I've tried a different approach to preserve the StringSlice alias type of []string at #102 by using global maps

[edoardottt]

@edoardottt I've tried a different approach to preserve the StringSlice alias type of []string at #102 by using global maps

Amazing !!!
I think this PR can be closed now :)