Small fix to example.
Make sort order definable on the command line.

With the -s flag, the user can submit a comma-separated list of fields
to sort on. Optionally, the field can be prefixed by asc: or desc: for
ascending or descending sort (ascending is default).

Example:

./sort -s desc:importance,desc:due,desc:priority,creation,description \
  todo.txt

Supported fields:

* priority
* due
* creation
* importance
* importance-no-wknd (skip weekends)
* description
* start (or t)
Invert sort order for priority.

The highest priorities have lower characters (first priority = 'A'), so
I think it's more natural to invert the sort. Sorting ascendingly means
lowest priorities first, then higher ones.

Given this additional complexity, the priority sort has been moved to a
separate function.