code: function Plugin() {
    // return value should be a callable function
    // with strictly defined options passed by markdown-it
    var self = function(md, options) {
      self.options = options
      self.init(md)
    }
  
    // initialize plugin object
    self.__proto__ = Plugin.prototype
  
    self.id     = 'render-readme-meta'
  
    return self
  }
docs: Constructor function