Refactored HTML formatter to use the extensible HTML formatter so multiple extensions can add extra details to the output.

- The zodyac/behat-extensible-html-formatter package must be installed for the custom HTML report to be active.
- Steps will now fail if there are differences unless the 'fail_on_diff' option is set to false.
- Added example time feature that will always fail (screenshots a webpage showing the current time).
Merge branch 'master' of https://github.com/jadu/BehatPerceptualDiffExtension