Add optional image diff view component (like github) #46

Add fade and swipe modes in addition to the existing 3-side gemini difference mode.
Also, github have slightly different difference mode, which implemented in the package too, and this mode perhaps can be ported in gemini-gui too (not for replace existing but in addition).

Demo: http://mxtnr.ru/image-diff-view/
Repo: https://github.com/a-x-/image-diff-view
Npm: https://www.npmjs.com/package/image-diff-view
Github image diff view modes: https://github.com/blog/817-behold-image-view-modes