docs: def _get_source(link):
      """
      Return source of the `link` whether it is filename or url.
  
      Args:
          link (str): Filename or URL.
  
      Returns:
          str: Content.
  
      Raises:
          UserWarning: When the `link` couldn't be resolved.
      """
      if link.startswith("http://") or link.startswith("https://"):
          down = httpkie.Downloader()
          return down.download(link)
  
      if os.path.exists(link):
          with open(link) as f:
              return f.read()
  
      raise UserWarning("html: '%s' is neither URL or data!" % link)