Google Analytics A/B testing support

[jamesrwilliams]

Add support for Google Analytics A/B testing. I recall this being done through URL vars but will have to research this more. An A/B test could enable/disable a feature flag.

[jamesrwilliams]

I've done some digging into this and the term I was looking for was "Redirect Test".

The idea here is that a user could setup a redirect test using a custom flag query argument triggering the feature flag plugin to display different content variations for the test.

This ties into the wide use of the query string things (see #12). Which leaves me with two questions really around features:

1. Add support for custom query strings that override teh default flag key.
2. Add support for a single query string to enable a set of feature flags.