code: func WithURLs(urls []string) Option {
  	return func(s *Source) {
  		if s.files == nil {
  			s.files = make([]*file, 0)
  		}
  		for _, url := range urls {
  			s.files = append(s.files, &file{OpenStringer: urlOpener(url)})
  		}
  	}
  }
docs: // WithURLs returns an Option which adds the slice of URLs to the set of data
  // sources a Source will read from. The URLs may be HTTP or local files.