text: Title: Store snapshot diff without storing previous snapshot
  Body: **Is your feature request related to a problem? Please describe.**
  
  Current API requires you you to store a previous snapshot before the next "diff" can be stored.
  
  For example
  
  ```python
      assert result == snapshot
      assert result_after_something_happens == snapshot(diff=0)
  ``` 
  
  If my test just depends on the diff snapshot (checking that the changes always remain the same) it's not possible to only snapshot the diff. 
  
  <!-- A clear and concise description of what the problem is. Ex. I'm always frustrated when [...] -->
  
  **Describe the solution you'd like**
  
  I'd be great if I could do something like this:
  ```python
      assert result_after_something_happens == snapshot(diff=result)
  ``` 
  