adding :default_call and :default options to class_attribute

Instead of starting up discussion about it on core list, wanted to submit so you could see a suggestion for class_attribute.

I know that currently default value can be done via just `self.name_of_attribute = (some value)`, but I need the ability to call a proc and get its value, where in my case the value is dependent on the class name where the module that uses class_attribute is located.

Suggestions welcome.