Support filtering tasks in "list" mode

[jrupac]

Inspired by #33 and #34. Tasky should support a flag to only show tasks in some/all task lists (depending on what's requested) that meet certain criteria.

E.g.:

$ tasky -l --filter=completed
$ tasky -l --filter=due_date

Options for the filter flag could include: {completed, not_completed, due_date, no_due_date}. These could also be composable (i.e., show only tasks not complete but with a due date).

As an added bonus, also support showing tasks due after a certain date:

$ tasky -l --filter=due_after=2019/01/01