Add an internal, override-able isVisible method

We've abstracted things into addressfield::hideField and addressfield::showField, but we still manually use the :visible selector in the main plugin method.

This will cause the plugin to break In cases where end-users have overridden the hide/show methods to match their markup.

Solution is to add an addressfield::isVisible method that matches the assumptions of our show/hide methods and use it in the main plugin instead of the :visible check.