Add contact_listname hook to handle special name order

I can modify the addressbook_name_listing variable via main config or preferences_list_hook but I have to change the rcube_addressbook class to use the modified value. Some contry use special [name orders](http://en.wikipedia.org/wiki/Personal_name#Name_order) and we can handle them with this hook.
I changed the hook name to contact_listname as proposed by thomascube in #127.