No tests though, not sure how to properly test this. Monkey patching the
datetime module doesn't appear to be working on Linux.

The number represents the position in the backing store, and can be
printed on the stdout to help the user to identify the correct task.

A view is a subset of the todo list where sorters and filters may be
applied.

At this point there are two filters:

* A grep filter that finds a pattern in a todo text
* A relevance filter, which selects todos that are relevant at this
  moment.

When setting a priority, a task may not be completed.
When a todo is completed, set priority to None.

A line is considered a todo item when it contains at least one
non-whitespace character.

Make sure the content is preserved byte by byte.

text() is the todo text without the tags. source() is the raw todo text.

A @ or + must be followed by at least one non-whitespace character to
qualify.