Implement sub-tasks logic

[sharkmu]

Be able to create sub-tasks, meaning that a simple task can contain smaller tasks (children).

Idea

• Display it in the list as well in a hierarchy style
• In the JSON
  • either the parent task struct could have an additional argument like children: []
  • or the task could become a list itself
  • probably the first one is better
• If you rm the parent task, then its sub-tasks also get deleted / marked as completed