[tmp] irb -r configurable
>> module Foo
>>   extend Configurable
>>   end
=> Foo
>> Foo.config do |config|
?>     config.setting_a = 'a'
>>     config.setting_b = 'b'
>>   end
=> "b"
>> Foo.setting_a
=> "a"
>> Foo.setting_b
=> "b"