Allow custom subscriber class to be a parameter

Hi,

Currently it's impossible to create a custom subscriber which class is a parameter.

This PR fixes it and allow to create subscriber like that:

    <parameters>
        <parameter key="my.query.paginator.subscriber.class">My\Subscriber\MyQuerySubscriber</parameter>
    </parameters>

    <services>
        <service id="my.query.paginator.subscriber" class="%my.query.paginator.subscriber.class%">
            <tag name="knp_paginator.subscriber" />
        </service>
    </services>