Calendar and todotxt compatibilty

Changes since 4.0.0:

• Fixed a few bugs related to calendar:

  • fixed panic when printing calendar for more than 1 year
  • when displaying more than 1 year, calendar does not abruptly stops printing after the first day of the last month in the range
  • calendar ranges like --calendar=-2y..-1y did not work - they displayed emptiness, now it is works as expected

• Fixed bug with date ranges that has none on one of its ends. e.g. --due=today..none. It should print all tasks which due date is today or later and tasks without due date set, but the old version displayed all the tasks

• New command-line flag priority-on-done (see documentation section Marking task completed and uncompleted) and configuration option global.priority_on_done. They define what to do with task priority when the task is completed. There are four modes: keep, erase, move, and tag - see details in the help.