title: How is Spree.config implemented?
body: <p>I'm studying Spree's source code and I came across the snippet.</p>
  
  <pre><code>Spree.config do |config|
    # Example:
    # Uncomment to override the default site name.
    # config.site_name = "Spree Demo Site"
  end
  </code></pre>
  
  <p>How is this being implemented? I cannot find Spree.config defined anywhere as a method. I'm guessing some meta-programming is involved. </p>