code: function(settings) {
        this.addSettings(settings);
  
        // register the service worker
        _serviceWorker.register(_settings['service-worker-url'], _settings['registration-options']).then(function(registration) {
          // Registration was successful
          if (_debugState) {
            console.log('Service worker registration successful with scope: %c'+registration.scope, _debugStyle);
          }
  
          // Send the settings to the service worker
          var messenger = registration.installing || _serviceWorker.controller || registration.active;
          messenger.postMessage({'action': 'set-settings', 'settings': _settings});
  
        }).catch(function(err) {
          // registration failed :(
          if (_debugState) {
            console.log('Service worker registration failed: %c'+err, _debugStyle);
          }
        });
      }
docs: Make this site available offline
  
  Can receive a settings object directly, or be configured by running addSettings() first.
  See Settings section of docs for details.
  
  #### Examples:
  ````javascript
  // Set up offline mode with a basic message
  UpUp.start({ content: 'Cannot reach site. Please check your internet connection.' });
  
  // Set up offline mode with the settings defined previously via addSettings()
  UpUp.start();
  ````
  
  @param {Object} [settings] - Settings for offline mode
  @method start