Extendable Configuration

[mike4git]

We need the Configuration of this bundle being extendable
so that other bundles can add or create their specific Converters and Populators
and offer them to be configured below Configuration key:

neusta_converter.

Concrete scenario

The pimcore-converter-bundle offers a LocalizedPropertyMappingPopulator which could be configured as simple as PropertyMappingPopulator.

So in the same way, it would be nice to just write in a Converter configuration:

neusta_converter:
  converter:
    my.converter:
      target: App\MyView
      properties:
        attribute1: srcAttribute1
        ...
+     localized_properties:
+       localizedAttribute1: srcLocalizedAttribute1

To allow that, a lot more things have to be prepared in converter-bundle Configurationclass.

[jdreesen]

I think we should use #41 as a base for this, as it already allows extending the config at certain points.