new_contents: from flask_table import Table, Col
  from flask_table.html import element
  
  
  class Item(object):
      def __init__(self, name, url):
          self.name = name
          self.url = url
  
  
  class ExternalURLCol(Col):
      def __init__(self, name, url_attr, **kwargs):
          self.url_attr = url_attr
          super(ExternalURLCol, self).__init__(name, **kwargs)
  
      def td_contents(self, item, attr_list):
          text = self.from_attr_list(item, attr_list)
          url = self.from_attr_list(item, [self.url_attr])
          return element('a', {'href': url}, content=text)
  
  
  class ItemTable(Table):
      url = ExternalURLCol('URL', url_attr='url', attr='name')
  
  
  def main():
      items = [
          Item('Google', 'https://google.com'),
          Item('Yahoo', 'https://yahoo.com'),
      ]
  
      tab = ItemTable(items)
      print(tab.__html__())
  
  if __name__ == '__main__':
      main()
  
subject: Add external_url_col example (#116)