Implement the `git stash` command.

`git stash save` = Stash: Save Changes
`git stash pop` = Stash: Apply (Pop)
`git stash apply` = Stash: Apply (Keep)
`git stash drop` = Stash: Delete

Edit: Might amend the commit to have "Stash: Delete (Drop)" so the fuzzy search works with people used to the drop keyword.