Register custom DQL functions

My use case is following, I want to register custom DQL function:

```yml
dql:
    string_functions:
        field: DoctrineExtensions\Query\Mysql\Field
```

Like documented in http://symfony.com/doc/2.3/cookbook/doctrine/custom_dql_functions.html.

But it is on default entity manager.

I wanted to fix it with:

```php
$em->getConfiguration()->setCustomStringFunctions($defaultConfig->getCustomStringFunctions());
$em->getConfiguration()->setCustomNumericFunctions($defaultConfig->getCustomNumericFunctions());
$em->getConfiguration()->setCustomDatetimeFunctions($defaultConfig->getCustomDatetimeFunctions());
```

in `Kayue\WordpressBundle\Wordpress\ManagerRegistry`, but there is no `getCustomStringFunctions()` in `Doctrine\ORM\Configuration`.

Do you have any better idea how we can fix this @kayue?
@kayue Good compromise, will submit PR.