Support Markdown task lists

## Feature Request Form
### Problem you are trying to address with this feature

In a Hypothesis annotation `- [ ]` at the start of a like is rendered as:

- `[ ]`

instead of as the intended:

- [ ] task list item

### Your solution

Add rendering of the above ASCII sequence as a checkbox, using for example a [GitHub](https://guides.github.com/features/mastering-markdown/#GitHub-flavored-markdown)/[GitLab](https://docs.gitlab.com/ce/user/markdown.html#task-lists)-flavored Markdown library.

### Add tags - did you add labels?
If this a customer requested feature, make sure to add that label.

Start by reviewing the annotation rendering path and the GitHub/GitLab task-list behavior linked in the issue. The work is done when an annotation line beginning with `- [ ]` renders as an unchecked task-list checkbox rather than literal `[ ]` text.

Written by the indexing model from the issue text.