Add a setting to append additional code to wsgi/fcgi deployment scripts

The existing `initialization` setting is handy for adding extra code to the wsgi files such as for loading celery, but for other purposes like New Relic integration, there's no way to wrap the `application` object with an agent. This change creates a new `deploy_script_extra` setting that appends its value to the .wsgi/.fcgi files.

Example usage:

    deploy_script_extra =
        import newrelic.agent
        newrelic.agent.initialize('/etc/newrelic.ini', 'app')
        application = newrelic.agent.WSGIApplicationWrapper(application)