code: function domain(url) {
    var cookie = exports.cookie;
    var levels = exports.levels(url);
  
    // Lookup the real top level one.
    for (var i = 0; i < levels.length; ++i) {
      var cname = '__tld__';
      var domain = levels[i];
      var opts = { domain: '.' + domain };
  
      cookie(cname, 1, opts);
      if (cookie(cname)) {
        cookie(cname, null, opts);
        return domain;
      }
    }
  
    return '';
  }
docs: Get the top domain.
  
  The function constructs the levels of domain and attempts to set a global
  cookie on each one when it succeeds it returns the top level domain.
  
  The method returns an empty string when the hostname is an ip or `localhost`.
  
  Example levels:
  
  domain.levels('http://www.google.co.uk');
  // => ["co.uk", "google.co.uk", "www.google.co.uk"]
  
  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