Add twentytwenty library to use in shots comparison

[mahmoudfelfel]

Add twentytwenty jquery plugin to use it for image comparison, as sometimes the diffs are simple and the one who testing can't notice them, so it is better to compare everything on one full-page view.

hint: i just made it work, the implementation is just css and plain jquery in gallery.html, i'm intending to enhance this later!

[jamesryanbell]

Thanks for this it looks good. The build seems to be failing due to an error in the package.json file, can you review the changes and fix the issue please so I can look at merge the pull request.

Thanks

[mahmoudfelfel]

Thanks @jamesryanbell i have fixed it, you can merge now.

[jamesryanbell]

Hi

Finally managed to get some time to test this out! It works and looks really great but there is a small bug. After the first dragging of the slider on the before and after image the height of the before and after container is reduced to the height of the page. This means you can no longer scroll down the image.

Thanks,
James