Active patterns

Sometimes its useful to set an active state to an list item when the `url` matches some pattern.

A good example is editing an user in which case the Users menu needs to be active

```php
Menu::handler('top', array('class' => 'nav'))
    ->add('/admin',      'Dashboard')
    ->add('/admin/user', 'Users')->activePattern('\/admin\/user\/\d\/edit')
    ->add('/admin/role', 'Roles')
```

will set an active class to the Users menu when on `/admin/user/5/edit` even if the particular menu item is not listed in the navigation