Add formatting overloads to HelpText.AddXXXLine(s)/Text (a la Console.WriteLine)

It would be convenient if `AddPostOptionsLine(string value)` and its sibling methods had formatting string (`String.Format` style) overloads such as `AddPostOptionsLine(string format, param args[])`.

This would allow code such as:

```cs
int ExitCodeSuccess = 0;
myHelpText.AddPostOptionsLine("{0} = Success", ExitCodeSuccess);
```