[5.0] improve app:name command to support sub-namespaces

right now if we want to fire `app:name` command with sub-namespaces, it will break `composer.json` files.

with this commit we can easily change between namespaces/sub-namespaces over and over again.

```
php artisan app:name Name\Space 
php artisan app:name Name\\Sub\\Space
```

either command will not breaking any files and namespace structure within `app` directory