Add @auth blade component

[m1guelpf]

I find myself writing @if(Auth::check()) in blade lots of times. This Pull Request adds a @auth blade directive to simplify this.

[taylorotwell]

I shouldn't be able to do @auth(api)... it should be @auth('api')

[akoury]

@taylorotwell @themsaid do you like the idea of having, for example, @auth($comments->count() > 1), in a way that works like an @if(Auth::check() && $posts->count() > 1) in a single command?, I know the parameter is currently a string for a guard, so we could check if it is not a string, then add the parameter as a second check