Init Flask Extension.

def init_app(self, app, config=None, statsd=None):
        """Init Flask Extension."""
        if config is not None:
            self.config = config
        elif self.config is None:
            self.config = app.config

        self.messages = self.config.get('EXCEPTION_MESSAGE', True)
        self.prefix = self.config.get('EXCEPTION_PREFIX', DEFAULT_PREFIX)
        self.statsd = statsd