code: function domain(url){
    var host = parse(url).hostname;
    var match = host.match(regexp);
    return match ? match[0] : '';
  }
docs: Get the top domain.
  
  Official Grammar: http://tools.ietf.org/html/rfc883#page-56
  Look for tlds with up to 2-6 characters.
  
  Example:
  
  domain('http://localhost:3000/baz');
  // => ''
  domain('http://dev:3000/baz');
  // => ''
  domain('http://127.0.0.1:3000/baz');
  // => ''
  domain('http://segment.io/baz');
  // => 'segment.io'
  
  @param {String} url
  @return {String}
  @api public