code: function contextRenderer (tpl, locals, options, noCache) {
      var finalLocals = merge({}, helpers, defaultLocals, this.state, locals)
  
      this.body = renderer(tpl, finalLocals, options, noCache)
      this.type = 'text/html'
      return this
    }
docs: Render function that attached to app context
  
  @param {String}  tpl     the template path, search start from viewPath
  @param {Object}  locals  locals, will merged with global locals and ctx.state
  @param {Object}  options options that pass to Pug compiler, merged with global default options
  @param {Boolean} noCache use cache or not