GitHub-style tasklists do not render checked boxes correctly

[shanebishop]

GitHub-style tasklists do not render checked boxes correctly, even when tasklists: true is set in the options.

For example, the following Markdown will render incorrectly:

 - [x] This renders without the checkbox showing as checked
 - [ ] This renders fine

I also see the following warning in the browser console on Chrome:

Warning: You provided a `checked` prop to a form field without an `onChange` handler. This will render a read-only field. If the field should be mutable use `defaultChecked`. Otherwise, set either `onChange` or `readOnly`.

Example render: