Eager load configuration

Added `Lotus::Application.load!` as configuration preloader. This will make possible the single file architecture.

```ruby
# config.ru
require 'lotus'

module OneFile
  class Application < Lotus::Application
    configure do
      routes do
        get '/', to: 'home#index'
      end
    end

    load!
  end

  module Controllers
    module Home
      include Lotus::Controller

      action 'Index' do
        def call(params)
          self.body = 'Hello'
        end
      end
    end
  end
end

run OneFile::Application.new
```

Reference #25