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.