Added the ability to define tracking, either through googleAnalytics or trackingCode

@SassDoc/owners Please review.

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Are we okay on key names (`googleAnalytics` and `trackingCode`)?

Or perhaps:

* `GAKey`
* `GATrackingKey`
* `googleAnalyticsTrackingKey`

And:

* `tracking`
* `trackingSnippet`

Thoughts?