Support flags with Enum type.

[alexpantyukhin]

For now the lib supports string and bool properties for commands flag. What if to support enum properties? The lib could validate the passed value and show the list of valid values in the help.
Any thoughts?

enum Color
{
        Red,
        Green,
        Blue
}
    
[Command("colorer", "Prints a color opinion.")]
class SayHelloCommand : ICommand
{
        [CommandArgument("c", "color", Description = "The color", DefaultValue = Color.Green)]
        public Color Color { get; set; }
        
        [CommandOutput]
        public IOutput Output { get; set; }

        public int Execute()
        {
                if (Color == Color.Red)
                {
                        Output.WriteInfo("Oh, no! Red color!");
                        return ReturnCode.Failure;
                }
			
                if (Color == Color.Yellow)
                {
                        Output.WriteInfo("Yellow color is better, but not enough!");
                        return ReturnCode.Failure;
                }
			
                Output.WriteInfo("Green color is awesome!!");
                return ReturnCode.Success;
        }
}