Allow extra fields on memory users config.

Description
Actually, if you use memory provider, you can only define few attributes (username, password, roles).
It will be nice, if you can define custom attributes.

Example
security.yml

security:
    providers:
        app_provider:
            memory:
                users:
                    adrien:  { password: pwd, roles: ['ROLE_USER'], extra: {gender: m} }

Somewhere.php
$user->getExtraFields()['gender'];
Class Symfony\Component\Security\Core\User\User already has $extraFields attribute.