Customize the tracker object

Hi,

This pull requests adds an extra configuration parameter to fully customise the tracker object. This is required especially if you need to add support for tracking across multiple domains. 

See: [Advanced Configuration](https://developers.google.com/analytics/devguides/collection/analyticsjs/advanced#customizeTracker) and [Domains & Cookies](https://developers.google.com/analytics/devguides/collection/analyticsjs/domains)

Thanks for this useful plugin!

Cheers,
Steven