Add option to prepend command in hook

[EriBloo]

Hi. I noticed a problem (at least in our setup) when running application in Laravel Sail/docker. All of commands I use go through Sail, but not git which is installed outside of container. When snippet is added to hook it won't run.

This commit adds prepend key to whisky.json which prepends snippet added to hook allowing users to have a little bit of control how Whisky is run.

Greetings! ;)