Added missing 'Cerebrate' section

Second part of resolution to Issue #75

Added missing Cerebrate config section to resolved the following error:

```
warning: Warning (2): in_array() expects parameter 2 to be array, null given in [/var/www/cerebrate/src/Controller/Component/Navigation/sidemenu.php, line 130]
Request URL: /users/login
Referer URL: http://127.0.0.1:8000/users/login?redirect=%2F
Client IP: 127.0.0.1
```

config/app_local.example.php

@@ -99,4 +99,8 @@ return [
             'url' => env('EMAIL_TRANSPORT_DEFAULT_URL', null),
         ],
     ],
+    'Cerebrate' => [
+        'open' => [],
+        'dark' => 0
+    ]
 ];