[Proposal] New 'visible' option on Navbar menus.

Sometimes when building the `Navbar` menu we need to hide/show some menus according to some specific criteria, specially when dealing with login/logout forms and etc.. Now, I find it quite tedious to do it this way, for instance:

````php
$menus = array(
	array(
		'label'		=> 'Always',
		'url'		=> '#'
	)
);

if (Auth::guest()) {
	$menus[] = array(
		'label'	=> 'Login',
		'url'	=> '#'
	);
}

Navbar::create()
	->with_menus($menus);
````

So I'd like to suggest we add `'visible'` as one of the menu attributes, so that we could do something like:

````php
Navbar::create()
	->with_menus(array(
		array(
			'label'		=> 'Always',
			'url'		=> '#'
		),
		array(
			'label'		=> 'Login',
			'url'		=> '#',
			'visible'	=> Auth::gest()
		)
	));
````

This way, the item would only be output if the `'visible'` attribute evaluates to `true`.

I could work on an implementation if you guys like the idea.