I18n - Allow custom translation loaders to be injected via module config #6244

This change allows modules to add their own translation loaders to the I18n translator via config like this:
```php
'translator_loaders' => array(
    'factories' => array(
        'my_new_loader' => 'ZendI18nDoctrineLoader\Factory\LoaderFactory',
    )
),
```
Before there was no way to add translation loaders without defining your own new translator service. See the ticket for more details: https://github.com/zendframework/zf2/issues/6244
I used the same doc block on this that is used in the rest of the framework for provider interfaces. Examples include the ControllerPluginProviderInterface and FilterProviderInterface among many others.