Add difftastic

[hiecaq]

Brief summary of what the package does

difftastic is a special major mode similar to diff-mode, which integrates the command line tool with the same name.

Direct link to the package repository

https://github.com/pkryger/difftastic.el

Checklist

Assume you're working on mpc mode:

• byte-compiles cleanly
• M-x checkdoc is happy. Don't manually write (provide 'evil-collection-mpc), M-x checkdoc can do it automatically for you
• define evil-collection-mpc-setup with defun
• define evil-collection-mpc-mode-maps with defconst
• All functions should start with evil-collection-mpc-