Initialize the extension. Configuration will be 
        obtained from ``app.config['MARROWMAILER_CONFIG']``. If no 
        configuration is found the mailer will be configured to 
        send emails asynchrously via SMTP on localhost without 
        authentication. The created ``Mailer`` instance is written
        to ``app.marrowmailer``.

def init_app(self, app):
        '''Initialize the extension. Configuration will be 
        obtained from ``app.config['MARROWMAILER_CONFIG']``. If no 
        configuration is found the mailer will be configured to 
        send emails asynchrously via SMTP on localhost without 
        authentication. The created ``Mailer`` instance is written
        to ``app.marrowmailer``.'''
        if not hasattr(app, 'extensions'):
            app.extensions = {}
        mailer = BaseMailer(app.config.get('MARROWMAILER_CONFIG') or self.default_config)
        app.extensions['marrowmailer'] = mailer
        app.marrowmailer = self