example_config.rb

class Foo
  class << self
    attr_accessor :configuration
    def config(value=nil)
      if block_given?
        yield configuration
      elsif value
        configuration[value]
      else
        configuration
      end
    end

    private

    def configuration
      @configuration ||= {}
    end
  end
end

Foo.config do |c|
  c[:foo] = 'bar'
end

puts Foo.config
puts Foo.config[:foo]

Foo.config[:bar] = 'bazz'
puts Foo.config