Nest list items via keyboard #349

Currently when user insert checkbox list, the items cannot be nested.
In case user focus cursor at start of the text and press Tab key, the item converts to structure equal to markdown

- [ ] foo
- - [ ] bar

So when foo will be checked, the bar will not.

User must be able to nest bar into foo like that

- [ ] foo
  - [ ] bar

It must be able via Tab key and Ctrl + ]