Feature: Support ToDo Syntax

[dschier-wtd]

Hi,

thank you for the awesome tool. Using it on a daily basis. It would be really nice to see improvements for the preview parser to support todo syntax like github (and other tools) do:

- [ ] open item
- [x] done item
- [ ] item
  - [ ] subitem
  - [ ] subitem

which will parse to:

• [ ] open item
• [x] done item
• [ ] item
  • [ ] subitem
  • [ ] subitem