Added a toggle tag, which can be used to toggle the time calculation

Use the toggle tag to toggle the time calculation on and off (like a stopwatch) for the tasks @lasted calculation. Here's an example:

TimeTests:

```
✔ two toggle middle @started(14-10-12 12:00) @toggle(14-10-12 12:30) @toggle(14-10-12 14:30) @done (14-10-12 15:25) @lasted(1:25)
✔ one toggle start @started(14-10-12 12:00) @toggle(14-10-12 12:00) @done (14-10-12 15:27) @lasted(0:00)
✔ one toggle end @started(14-10-12 12:00) @toggle(14-10-12 15:28) @done (14-10-12 15:28) @lasted(3:28)
✔ one toggle middle @started(14-10-12 12:00) @toggle(14-10-12 13:00) @done (14-10-12 15:58) @lasted(1:00)
✔ three toggles @started(14-10-12 12:00) @toggle(14-10-12 12:30) @toggle(14-10-12 14:30) @toggle(14-10-12 15:00) @done (14-10-12 16:05) @lasted(1:00)
✔ four toggles @started(14-10-12 12:00) @toggle(14-10-12 12:30) @toggle(14-10-12 14:30) @toggle(14-10-12 15:00) @toggle(14-10-12 16:00) @done (14-10-12 16:07) @lasted(1:07)
✔ zero toggles @started(14-10-12 12:00) @done (14-10-12 16:09) @lasted(4:09)
```