Use Checkboxes for sub-tasks

[ChanderG]

We don't have support for Checkboxes yet in the system.

The steps (which need single key mappings) are:

1. Edit a todo and manually create a list and add entries to it of the form "[ ] description"
2. Use C-c C-c to toggle checkbox.
3. Add [/] or [%] to the task title manually.
4. Run C-c # to update the task counter.

https://orgmode.org/manual/Checkboxes.html