Enhancement/merge extension config

Extension configs previously did not seem to merge correctly. For example there didn't seem to be a way to override `default_db` without creating a whole new `DocumentManagerFactory` which seems a little to verbose.

I thought something like this would work?
```php
$manifest = new Manifest(
            [
                'extension_configs' => [
                    'extension.serializer' => true,
                    'extension.odmcore' => [
                        'default_db' => 'test_db'
                    ]
                ]
            ]
        );
```

Let me know if it's completely wrong.