Introduce #default method to set application options

Working on Padrino extensions with a lot of configuration options I ended up with calling tons of

```ruby
        app.set :login_url, '/login'             unless app.respond_to?(:login_url)
        app.set :login_model, :account           unless app.respond_to?(:login_model)
```

Since we are claiming to be modular do you think it would be handy to have `app.default` method to be able to set these options if they are not already set?