Generate a due date directly from the add command

[fnuttens]

The idea is to implement a new option (e.g. --due) to the add command. It would append a due date to the task, like in the following:

(A) 2020-05-18 search for new feature ideas +todotxt-rs due:2020-06-18

It could accept either an absolute date (e.g. --due 2020-06-18) or a relative time (e.g. --due 1 month).