Implement hit diff command with pretty diff

[chshersh]

This command will show pretty diff output like in the image below:

The command to do this:

git diff HEAD~1 --color=always | diff-highlight | less -rFX

However, diff-highlight requires installation. Instructions:

1. Ubuntu: https://qiita.com/SakaiYuki/items/2814d417d0bec59046bb
2. Macos: https://www.viget.com/articles/dress-up-your-git-diffs-with-word-level-highlights/

This to look for:

1. Check whether diff-highlight is on path and call ordinary diff