Deep linking does not update menu item highlights

[goblindegook]

Navigating straight to a deep link does not update the menu appearance.

For example, pointing your browser to http://b3.dev/category/images does not highlight the menu items in the Category > Images path.

The onMenuItemUpdate event handler in the new MenuItemActivation behavior should also look at the menu item's object (containing the post or term ID), object_parent and object_type to determine whether the menu should be highlighted when the application loads.

[goblindegook]

At the moment, menu item state is updated on navigation events.

Instead, we should trigger the update when an archive or post view is loaded. Because the menu model contains references to object types and IDs, we can easily locate the corresponding menu item(s) to highlight. (Only a minor change is required to the MenuItem behavior for this.)

[goblindegook]

Fixing in #52. Still need to work around the possibility that the menu isn't available on view load.