Change the initialism selector

Created by: zlatanvasovic

It would be nice to change abbr initialism selector from abbr.initialism to .initialism.

Changing selector will make class usable as mixin. That also doesn't require changing markup, so it can be in a patch relase (correct me if I'm wrong).

If using .initialism affects global namespace, please change class name to .abbr-initialism in v4.

Thanks in advance.