configurable.rb

module Configurable
  def self.with(*attrs)
    not_provided = Object.new

    config_class = Class.new do
      attrs.each do |attr|
        define_method attr do |value = not_provided, &block|
          if value === not_provided && block.nil?
            result = instance_variable_get("@#{attr}")
            result.is_a?(Proc) ? instance_eval(&result) : result
          else
            instance_variable_set("@#{attr}", block || value)
          end
        end
      end

      attr_writer *attrs
    end

    class_methods = Module.new do
      define_method :config do
        @config ||= config_class.new
      end

      def configure(&block)
        config.instance_eval(&block)
      end
    end

    Module.new do
      singleton_class.send :define_method, :included do |host_class|
        host_class.extend class_methods
      end
    end
  end
end

test.rb

require_relative 'configurable'

class Test
  include Configurable.with(:first, :second, :third)
end

Test.configure do
  first "First"
  second "Second"
  third { "#{first} - #{second}" }
end

puts Test.config.first #=> First
puts Test.config.second #=> Second
puts Test.config.third #=> First - Second