Pass relative to absolute date conversion an optional offset.

If offset is not defined, it defaults to today (previous behavior).
Add new subcommand for postponing tasks.

It sets the due date to an absolute date or a relative pattern, for one
or more todos.

Example:

    todo.sh postpone 1,3,8 1w

This postpones tasks 1, 3 and 8 with one week, but only those that have
a due date set.