Setup the plugin from an application.

def setup(self, app):
        """ Setup the plugin from an application. """
        super().setup(app)

        if isinstance(self.cfg.template_folders, str):
            self.cfg.template_folders = [self.cfg.template_folders]

        self.cfg.template_folders = list(self.cfg.template_folders)
        if not self.cfg.loader:
            self.cfg.loader = FileSystemLoader(
                self.cfg.template_folders, encoding=self.cfg.encoding)

        self.env = jinja2.Environment(
            auto_reload=self.cfg.auto_reload,
            cache_size=self.cfg.cache_size,
            extensions=self.cfg.extensions,
            loader=self.cfg.loader,
        )
        self.env.globals['app'] = app

        @self.register
        @jinja2.contextfunction
        def debug(ctx, value=None):
            """ Debug current context to template. """
            return jinja2.filters.do_pprint(value is None and ctx or value)

        @self.filter
        def jsonify(obj):
            return dumps(obj, ensure_ascii=False, encode_html_chars=True)