[Feature] support subtasks

[huyz]

If the current task is sent to Things, then bundle any indented sub-tasks below as lines in the checklist for that task in Things.

Since Things only supports one level of subtask, there would need to be a decision of whether to (a) flatten the subtask hierarchy or (b) visually indent each subtask with spaces in Things.

in other words,

• level 1
  • level 2
    • level 3
  • level 2.2

could become in Things:

a)

• level 1
  Checklist:

  - [ ] level 2
  - [ ] level 3
  - [ ] level 2.2
  

or
b)

• level 1
  Checklist:

  - [ ] level 2
  - [ ]   level 3
  - [ ] level 2.2
  

I prefer the (b) option but I could see an argument for (a).