function domain(url){
  var host = parse(url).hostname;
  var match = host.match(regexp);
  return match ? match[0] : '';
}
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