Adding new link types: <current> and <column>

The following patch adds two new special menu items:

: generates a link to the current page
: similar to a no-link but has a different class (column) and allows a title of for blank titles.

The link is being used on sites for parent menu items where clicking the parent just keeps you on the same page. This is needed for sites that have theming that requires the items to be a link but you still don't want to leave the page if it's clicked.

The link is used on our site for making multi-column drop-down menus. We use css to float the "column" class to the left so that each submenu that has the column class forms a new column. The title of the column is rendered at the top of the column unless is used for the title.

You'll see that this is pretty minor changes to the existing module. At some point it might be good to figure out how to support plugins with this module (e.g. ctools) so that it's easier to extend the features of this module.