Add negative streak (#110)

[inad9300]

As noted in #110, a "negative streak" would be really helpful in knowing what you should be paying closer attention to every day. I have extended the existing code in the most straightforward way to achieve this.

Note that old streak values have not been recalculated. I think this is not too important because negative streaks don't show up in any of the histories, and it is rare that people go back many days manually just to see this kind of info (and even if they do, I don't think it's a big deal). That said, of course feel free to do something about this.

Please, someone test the code and verify that everything is behaving as expected.

[nbeloglazov]

Any update on this pull request? I'd love to have negative strikes in the app.