Add `git snapshot`

Take a snapshot of your current working tree without removing the changes from your tree. This is handy for refactoring where you can’t quite fit what you’ve done into a commit but daren’t stray too far from now without a backup.

Taken from [Handy Git tips to stop you getting fired](http://blog.apiaxle.com/post/handy-git-tips-to-stop-you-getting-fired/)