[WIP] Add git stash option to rubocop precommit

This is a work in progress.

I have added a stub implementation of stashing changes before a rubocop validation. 

One problem that could arise is that the `stash pop` failure. Looks like it is possible to simulate a `git stash pop --force` >> https://coderwall.com/p/anxp0g

Also I have checked that the `ruby-git` gem does not implement `git stash pop`.