Foundational concept design: task attributes

**RFC link:** [Tasks and Attributes](https://github.com/ForNeVeR/praefectus/blob/master/docs/rfcs/tasks-and-attributes.md)

We'll need a set of the most basic task management concepts: tasks and their attributes, including:
- basic attributes: task title, creation date
- extended attributes (various Taskwarrior stuff, including Taskwarrior task UUID, priority, tags etc.)
- volatile attributes (Taskwarrior)
- cross-task dependencies
- ~~an algorithm to detect dependency loops (Taskwarrior disallows them, but they still may be introduced if a conflict occurs)~~

Depends on:
- [ ] #22
- [ ] #39