[WIP] Support checkboxes/task lists

[willdurand]

Purpose

The goal of this PR is to support task lists (à la GitHub). This
PR implements two parts:

1. The rendering of task lists in the Preview component
2. Two-way binding to udpate task lists, which means you can either
   modify the markdown itself or click on the task list items
   in the preview panel!!!

Features

• Render task lists in the preview panel
• Implement a basic "task list item" finder in a Markdown content
• Allow to toggle task list items (that is, check/uncheck)
• Write tests for the documents module
• Listen to checkbox clicks in live (so that we can add/remove
  items)
• Add data attribute on each checkbox
• Wire everything and make it work
• Even more tests

Example