* Add ability for interactively explore project when calling `terka show
project` command.
* Simplify the codebase:
* Add `is_stale`, `is_overdue` and `completion_date` properties to
Task (and remove their calculation from `printer`)
* Add `total_time_spent` property to project
* Make `TerkaTask` view neater, remove not working `Comment` section
* Add `formatter` module to extract formatting operations