Handle "pre add job" parameters

[bleutzinn]

Some services need parameters set before it's "job" is added to tarteaucitron. For example:

        <script type="text/javascript">
        tarteaucitron.user.googlemapsKey = 'API KEY';
        tarteaucitron.user.mapscallback = 'callback_function'; // optionnal - for markers
        (tarteaucitron.job = tarteaucitron.job || []).push('googlemaps');
        </script>

This is not handled yet.