function convert(node) {
                node.md = (node.md && node.md.join('')) || ''
                return (converter[node.name] || converter['default'])(node) || ''
            }
@param html {String} the html to be onverted
@param options {Object}
  {
     converter {String} which converter you choose
     overides {Object<String, Function>} override converter behavior, for example:
         {
             br: function (node) {
                 return `\n\n` // let br tag break twice
             }
         }
  }