title: Require all files in sub-directory
body: <p>I have the following directory tree.</p>
  
  <pre><code>- app.rb
  - folder/
    - one/
      - one.rb
    - two/
      - two.rb
  </code></pre>
  
  <p>I want to be able to load Ruby files in the folder/ directory, even the ones in the sub-directories. How would I do so?</p>