Add public method GetModel to CommandApp

[phil-scott-78]

Bit of a weird one, I just want a public method on CommandApp that would return ICommandModel. I was trying to access this from outside of the app for documentation generation reasons and it would be quite handy to grab this programmatically at runtime without needing to shell out to grab the XML

  public ICommandModel GetModel()
    {
        // Build the model straight from the configuration, without running the app or
        // injecting the built-in `cli` branch. Also validates, so a misconfigured app
        // throws here just as it would on Run.
        return CommandModelBuilder.Build(_configurator);
    }