delete useless assertion
tasks can be completed without completion date

Althrough strictly the todo.txt format mandates a completion date it is
impractical. Task now has a completed flag with is set to the presence
of '^x ' in a task line. Then the completion date is read, if present.
hide duplicate priorities in task listing

As tasks may share a priority their priority is not printed for each
task.

Prioritized tasks are now separated from non-prioritized tasks by an
empty line.