feature request: selctor-based link crawling

A "crawlSelectors": [] property will be added to each scenario in the backstop.json config file. For each matching crawlSelector, a collection of matching elements will be returned. For each matching element, the original URL will be reloaded and the current element index will receive a click event. The resulting page or state will be loaded and tested inheriting all the test properties (except crawlSelctors) from the parent scenario.

This would enable rudimentary testing of various page links, pop-up and control states.