[BUG] Start command is adding one more day than expected

I found a bug where the count was always putting an additional day.

It became clear when I dit a `start 1`, and it printed:

`Started new wordtrack period (1 days including today) with the goal of writing 50000 words. Good luck!`

But the content on the wordtrack.txt was:

```
startdate: 2019-10-12
enddate: 2019-10-13
wordgoal: 50000
day 1: 0
```

I'll create a PR to fix this