Add a minimum time criterion

[joeljfischer]

Fixes #2

Proposed Changes

• Add an optional minimum time criterion between the first active positive event and the current time

[joeljfischer]

Regarding the time frame, I would prefer something more flexible like what I currently have implemented. In my app I have a delay of a week, for example. I understand the current naming being unclear. If you wanted to go the direction of using a Bool, that's fine, I already have a fork that I'm using.

As far as the first event in the list vs. the first app open, I think the first event in the list makes more sense. What I was trying to avoid was a situation where the user opens the app, then doesn't use it for a month, then opens it again and starts using it. I'd prefer to see the later app open as the new "first time it's used".