Custom adjusted bounce rates

We have sets of pages for which it's useful to implement Adjusted Bounce Rates analytics to get a better idea of how much user engagement our articles are generating, per Google's documentation of the concept (http://analytics.blogspot.ca/2012/07/tracking-adjusted-bounce-rate-in-google.html). This PR adds simple support for specifying time intervals at which tracking events should be generated for calculating such adjusted bounce rates.