Add support for a data-attribute to close open submenus

[mrwweb]

Now that it's possible to close an open submenu from anywhere in the DOM (#18), it would be a very nice developer convenience to declaratively use that feature.

So my current thought is this:

<button data-close-clicky-menus="menu-id">Close Open Menu</button>

Where "menu-id" is the ID of a valid clicky-menu instance. If that were in the DOM when the script first executes, I could find those instances and then automatically handle closing submenus when the button is clicked, reducing requirements for custom code.

[ajplopez]

This would be amazing. I'm starting to have close menu buttons within mega menus more often so that would make life a lot easier.