Add a new arg for custom ID attributes with wp_page_menu()

[iandstewart]

The attached patch adds a new arg to wp_page_menu() for setting custom ID attributes (right now you can only set a Class--incorrectly noted as the ID in the docs).

Since wp_page_menu() is the default fallback for wp_nav_menu() this would be potentially super-helpful for marrying up with wp_nav_menu()'s container_ID.

Related #13998