Verify that specs don’t leave unpopped stashes

@charlierudolph this implements #80 
would `finish-with-non-empty-stash` or something along that line be a a better tag name?

And would be nice to write a helper that returns the length of the stash
```ruby
def stash_size
  run("git stash list | wc -l")[:out].to_i
end

After '~@allow-stash-afterwards' do
  expect(stash_size).to eql 0
end
```