Add 'auto' to the Google Analytics tracker create method.

Google Analytics supports [automatic cookie domain configuration](https://developers.google.com/analytics/devguides/collection/analyticsjs/domains#auto), and this is now officially part of the recommended [GA code snippet](https://developers.google.com/analytics/devguides/collection/analyticsjs/).

When this parameter is omitted (as it was prior to this pull request), Google Analytics will default to using `location.hostname`, which will include the subdomain. This prevents websites with multiple subdomains from being able to automatically track unique users across those subdomains.
@philipwalton the suggestion was only for the code from the `docs`, not `index.html` (I apologize if I wasn't clear enough).