Add 'offset' parameter and 'ancestral' boolean to wp_nav_menu

[signyourbikeout]

Since we can now call wp_nav_menu multiple times and there is the very useful 'depth' parameter, I believe adding an 'offset' parameter would be very beneficial in many situations. This would allow you to start at the children of the main pages for example and combined with depth it becomes very powerful. An 'ancestral' => 'true' option could output only menu items that the current page is a descendant of. Currently if you want to display a main menu in the header and then a submenu on page.php you either need a tricky custom walker or terrible CSS 'visibility:hidden' or 'display:none' situations.