How is Spree.config implemented?
===

I'm studying Spree's source code and I came across the snippet.

    Spree.config do |config|
      # Example:
      # Uncomment to override the default site name.
      # config.site_name = "Spree Demo Site"
    end

How is this being implemented? I cannot find Spree.config defined anywhere as a method. I'm guessing some meta-programming is involved. 