Automatically showing help using default command

[krisrok]

As some users pointed out in #702 it'd be nice to also show the help/usage when using a default command like here:

var app = new CommandApp<DoStuffCommand>();
app.Run(args);

When run without any arguments given it currently only shows:

Error: Missing required argument 'myArgument'.

It only works when adding it as a regular command

var app = new CommandApp();
app.Configure(c => c.AddCommand<DoStuffCommand>("run"));
app.Run(args);

But as it is the only command this name is totally redundant and the help showing up is unneccessarily hard to read:

USAGE:
    App.exe [OPTIONS] <COMMAND>

OPTIONS:
    -h, --help       Prints help information
    -v, --version    Prints version information

COMMANDS:
    run <myArgument> [myOtherArgument]

Instead of just showing the missing argument error I'd much rather like it to automatically (or switchable by config) render the help like when called with -h.

Forgive me if I overlooked how to do this correctly if there's already a way.