Restore menu item IDs

[natejacobson]

I'm very happy with our much reduced class assignments on menu items, but in anticipation of in-page navigation (and navigation that highlights with scrolling), we should restore the IDs we are stripping with the spine-abridged_menu_classes function.

function spine_abbridged_menu_classes( $classes ) {
if ( in_array( 'current-menu-item', $classes ) || in_array( 'current_page_parent', $classes ) ) {
    return array( 'current' );
}

return array();
 }

I did take a stab at this, but without success.

[jeremyfelt]

Should we still think of doing this? Similar to 9c4a40d, we can just append current instead of stripping everything else.

[philcable]

Closing per discussion with Jeremy.