Added formatFailure method

Attempting to use this library as part of https://github.com/shakyShane/browser-sync - it works great, but I needed this modification...

I do not want/need the XPath to an element, but rather some other info about the dom node (that is not available through reporters), so this PR allows me access to the DOM node so that I can format the failures to suit my needs.

After months of trying countless Lib's - yours is by far the most stable/reliable & I'd be greatful for any feedback on this addition.