HelpOption display bug

[tomrus88]

Describe the bug
Help text is missing option name when using default short help template like this

[HelpOption("-h")]
abstract class SomeCommandBase
{
    protected virtual int OnExecute(CommandLineApplication app)
    {
        app.ShowHelp();
        return 1;
    }
}

[Command("somecmd")]
class MyApp : SomeCommandBase
{
}

You get following help texts:

Run 'appname [command] --' for more information about a command.

instead of

Run 'appname [command] -h' for more information about a command.

and

Specify - for a list of available options and commands.

instead of

Specify -h for a list of available options and commands.

Issue seems to be somewhere in those lines:
https://github.com/natemcmaster/CommandLineUtils/blob/master/src/CommandLineUtils/HelpText/DefaultHelpTextGenerator.cs#L281
https://github.com/natemcmaster/CommandLineUtils/blob/master/src/CommandLineUtils/CommandLineApplication.cs#L994

Also please consider making CommandOption.UnderlyingType and DefaultHelpTextGenerator.ExtractNamesFromEnum protected to make creating custom help generators easier.

[natemcmaster]

Looks like a simple bug. I would be happy to accept a pull-request to fix the issue.

Re: CommandOption.UnderlyingType - this one is intentionally internal because I plan to refactor how it is used relatively soon. I don't want to risk someone taking a dependency on this API by making it accessible.