Implement delta image creation function

[cryptaliagy]

As discussed in #27 and #28, there will need to be some way to compare the two images (the baseline and the newly generated one), and confirm that the image is the same image.

This issue covers the implementation of the delta image creation as well as verification that the screenshots match.

Originally I thought it might be best to have the plugins handle the logic for verification themselves, but now I think it might be wiser to let the VisualParityValidation object handle that since it will be shared across all storage methods, Instead, the plugins can be supplied with only the baselineID of the image and then they will return the image itself. The above tickets will be edited to reflect this.

Ideally, the number of dependencies added to the project to enable this should be 0, but it might be wise to use an image processing library to compare the images and to produce the delta. If that is the case, the Pillow library (https://python-pillow.org/) might be useful

[cryptaliagy]

Pillow is a library with no other dependencies, which as far as things go is pretty good. It also uses C++ code to do all the image processing, it seems, which might be a smarter plan than trying to do it on my own