[New Feature Request] Test image could compare the differences with multiple baseline images

Hello Garris,

We really like the Backstop for visual testing solution.

Considering the A/B testing is more and more popular for nowadays websites, simply using ignore elements feature cannot fullfill the A/B testing sites.

Here, we would like to request to add a new feature for BackstopJS:
**The test image could compare the differences with multiple baseline images.**

You are very welcome if you have any questions.

Cheers,
Ken