Store snapshot diff without storing previous snapshot

[freshollie]

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

    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.

Describe the solution you'd like

I'd be great if I could do something like this:

    assert result_after_something_happens == snapshot(diff=result)

[noahnu]

This sounds reasonable to me. Would you be interested in trying your hand at contributing with a pull request? Otherwise, I'll add this to my backlog.

[freshollie]

I'll have a go at a PR over the next week or so :)