「多套配置」是指：Cls.config_a.foo; Cls.config_b.foo？如果是这个意思，那么我粗糙地实现了一下：

# configurable.rb
module Configurable
  module Enable
    cattr_accessor :current_config

    def method_missing(method, *args)
      cattr_accessor method
      define_singleton_method method do
        self.current_config = method

        config = instance_variable_get("@#{current_config}_config")
        config.nil? ? self : config
      end
      send method
    end
  end

  def self.with(*attrs)
    # 使用配置属性定义匿名类
    config_class = Class.new do
      attr_accessor *attrs
    end

    # 为可混入的类方法定义匿名模块
    class_methods = Module.new do
      define_method :config do
        config_var = "@#{current_config}_config"
        instance_variable_set(config_var, config_class.new) if instance_variable_get(config_var).nil?
        instance_variable_get(config_var)
      end

      def configure
        yield config
      end
    end

    # 创建并返回新的模块
    Module.new do
      singleton_class.send :define_method, :included do |host_class|
        host_class.extend class_methods
      end
    end
  end

end

# my_config.rb
class MyConfig
  extend Configurable::Enable
  include Configurable.with :foo
end

# Usage
MyConfig.config_a.configure do |config|
  config.foo = 'bar'
end
MyConfig.config_b.configure do |config|
  config.foo = 'foo'
end

puts MyConfig.config_a.foo
puts MyConfig.config_b.foo