config.rb

module Rack
  class Foo
    def initialize app
      @app = app
    end

    def call(env)
      [200, { 'Content-Type' => 'text/html' }, 'OK']  
    end
  end
end

use Rack::Foo

app = lambda { |env| [200, { 'Content-Type' => 'text/html' }, 'Hello World'] } 
run app