Hierarchy

hello @NayamAmarshe After using your please CLI for a few months daily, the only shortcoming I felt encountering for my usage would be detailing some tasks with a few sub-tasks.

So naturally I forked the project and added this functionnality, please find linked to this enhancement issue my branch, and if so inclined, you may review it and eventually merge it.

https://github.com/vchalmel/please/tree/hierarchy c.f. https://github.com/NayamAmarshe/please/issues/55