[ ] a
[ ] b
- [ ] c
- [ ] d

If user is careless, forgetting to put "- " before "[ ]", TODO list could get buggy.

Steps to reproduce bug:
- on parsed MD page of list above, tick "c" to note that "c" is done.
- Now, edit the file.

Expect: will be "[ ] a" and "- [x] c"
Result: will be "[x] a" and "- [ ] c"