Taskwarrior integration

I want checktimer to become fully integrated with [Taskwarrior](https://taskwarrior.org/). It should eventually store its data in Taskwarrior database.

- [x] (#40) add configuration file support (and store `task` and `checktimer.csv` paths there)
- [ ] (#41) add Start button to select task from the Taskwarrior database
- [ ] import currently active task from Taskwarrior at start
- [ ] store Taskwarrior task UUID in the checktimer database
- [ ] Start button should execute `task start` for the corresponding task
- [ ] Stop button / function should execute `task stop` for the currently started task
- [ ] store the duration data as a Taskwarrior [User Defined Attribute](https://taskwarrior.org/docs/udas.html) (and register the attribute automatically when needed); get rid of custom CSV file

After implementation of all those tasks, Taskwarrior installation will be required for checktimer to work.