Use BackstopJS to diff/test screenshots from other software (e.g. Angular Protractor)

Fantastic work on this project! I love the [new UI](https://github.com/garris/BackstopJS/issues/650).

I'd like to be able to use BackstopJS to diff screenshots taken by other software, rather than using BackstopJS's screenshot capturing features.

Integrating BackstopJS testing into a modern Angular application seems to require a lot of configuration. Rather than redefining all the paths, scenarios, mock content, and testing procedures again for BackstopJS, I'd like to simply take screenshots during my existing e2e tests (running in [Protractor](http://www.protractortest.org/)). Protractor already makes this easy, and configuration is fairly standard across the Angular ecosystem.

Once protractor has saved the screenshots to a temporary directory, I'd like to be able to pass those screenshots to BackstopJS for comparison/testing/approval in a separate npm task.

Has anyone tried this setup before? (Or have other recommendations for integrating BackstopJS into an existing e2e test suite?)

Related issues:
#622
#293