Add filter for attributes on menu item links

[simonwheatley]

I find myself needing to change the attributes of the link element of each menu item to add some classes. Currently I can do this by extending and substituting Walker_Nav_Menu, but that the start_el method I replace is fairly long and I'm nervous of changes to core breaking my kit in the future. Much better to be able to hook a filter, IMO.

I propose a new nav_menu_a_attributes filter in the start_el method of Walker_Nav_Menu, as per the attached patch.

In the patch I've also changed the way the attributes are built up within start_el so they work with a named array, as it is easier for plugin authors to then parse the attributes as an array than if they are passed a long string of HTML attributes. Hope that makes sense.

Looks good to me.