Support BackstopJS Reporting Dashboard

[greenyouse]

Hi, I'm not sure how active this project is but I thought it might be helpful to add the BackstopJS reporting dashboard to this tool. After doing a run with wdio, it could write html assets for the test results and launch the report UI to show what passed and failed for visual differences. This would help devs and QA teams review their visual diffs a bit faster.

I've put together a branch with basic support and full test coverage. I can have a PR out tomorrow-ish after wrapping some things up.

[greenyouse]

Here's the PR for the issue #99

Thanks for the work you've done building and maintaining this library! If you'd like me to refactor any of the code I'd be happy to tweak things. I'm not as familiar with mocha so there might be some improvements I could make.

No problem if you'd rather not add BackstopJS support. It was more of a one-off idea but I thought it might help other teams using this library.