Subtasks support

[g33kex]

Thank you for this very nice task manager!
Having support for subtasks would be great.

Also, that would fix #22.

Something like this would be nice:

1. ☐ Task 1
3.     ☐  SubTask 1
4.         ☐ Subtask 2
5.     ●  Note
2. ✔ Task 2

Maybe when a task is checked, the parent task could be considered partially completed:

1. [-] Partially completed task
2.     [✔] Completed Subtask

Implementation wise that would probably mean to add a parent to each task so we have a tree like structure. I don't know much of Typescript though.