New behavior of option --todo

[OliverCi]

Change behavior of option --todo: If a node is explicitly marked with an icon denoting a task, do not add checkboxes to its descendant list items (unless they are also explicitly marked with a task icon). Provide new option --todo_all for old behavior.

Motivation: When a task icon has been explicitly set, subitems tend to be additional information, rather than subtasks.