[5.3] A notification can be broadcasted to custom channels

[shadoWalker89]

This PR will allow to specify the channels to broadcast to, upon overriding the broadcastOn() on a notification.

class ProductPurchased extends Notification implements ShouldQueue
{
    public function broadcastOn()
    {
        return [
            new PrivateChannel('sales'),
            new PrivateChannel('accountant'),
            new PrivateChannel('admin')
        ];
    }
}

[shadoWalker89]

On second thought maybe it would be better if we use routeNotificationForBroadcast() to define custom channels instead of overriding the broadcastOn(). Just to be consistent with the other Channels.

If you like we can even keep them both.
What do you think @taylorotwell ?