Preserve existing requirejs configuration

[huntc]

Our requirejs setup is now a little more considerate when establishing requirejs configuration i.e. it now considers any existing configuration.

This is necessary when requiring to pass configuration from the html template e.g.:

<script type="text/javascript">
    var require = {
        config: {
            'controllers/Application' : {
                tzs: '@routes.Application.tzs()'
            }
        }
    };
</script>

Prior to the fix, we were simply overwriting any existing configuration.