make Value implementations public

[DenKoren]

this makes possible to do the following:

flag := pflag.Flag{
	Name:      "sample-option",
	Shorthand: "s",
	DefValue:  "default value",
	Value:     pflag.NewStringValue("default value", new(string)),
	Usage:     "option you can add to cobra command with Flags().AddFlag()",
}

cobra.Command{}.Flags().AddFlag(&flag)

It allows one to keep the flag definition separately and use the same flag in several CLI commands or subcommands.

cobra has AddFlag() method, but, as far as I see, user can't just make Flag structure and use it. The thing that causes problems is Value field: it requites Value interface implementation, but you can't access default implementations available inside pflag. When Value field is nil, you face 'nil pointer dereference' error.

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