[5.0] Add The Ability To Register Subscribers

To use this you would simply add the class names you wish to register in the `$subscribe` property in the `App\Providers\EventServiceProvider` class. I've added the property to the class itself because I didn't want to make laravel/laravel dependent on this change. I also kept `$subscribe` singular to remain consistent with `$listen`.
Are you sure? Why are *listeners* registered in $listen? I'll keep it this way for consistency with the rest of the code in this way unless there's a good reason not to.