Add closure support to menus

[ThamanVelasquezBice]

I added support for the 'items' array to accept a closure. I tried to use just the closure without editing your code, but php gave me a segmentation fault.

'items' => [
    // This menu will be shown regardless of whether or not
    // the login system is enabled!
    function() {
        if(auth()->user()->hasRole('developer')) 
        {
             return ['Admin Stuff' => ['link' =>'some/cool/route', 'icon' => 'fa fa-user']];
        }
    },
    'Like Things?' => [
        'link' => '#',
        'icon' => 'fa fa-line-chart',
    ],
],