Output tasks using GitHub task lists

[RealOrangeOne]

This allows us to use more of GitHub's project features

https://docs.github.com/en/get-started/writing-on-github/working-with-advanced-formatting/about-task-lists

Fixes https://github.com/srobo/recurring-tasks/issues/73

[PeterJCLaw]

Nice, do you have a demo import of this somewhere you've tested with? (only needs to be a couple of tasks, no need for a whole tree)

[RealOrangeOne]

Untested yet, mostly opening as an RFC. Will test soon.

[PeterJCLaw]

Nice, do you have a demo import of this somewhere you've tested with? (only needs to be a couple of tasks, no need for a whole tree)

I've imported a test tree into https://github.com/PeterJCLaw/test/issues/817

[PeterJCLaw]

I've extended this with deeper task-list stuff. Example imports:

• https://github.com/srobo/test/issues/9
• https://github.com/srobo/test/issues/6
• https://github.com/srobo/test/issues/3