Issue title: Markdown checkboxes not rendering in markdown vis
 Issue body: Checkboxes are not converted to checkbox-inputs

```
 - [x]
 - [ ]
```

see https://discuss.elastic.co/t/checkboxes-markdown-not-displaying/58370

This is an upstream issue with marked (https://github.com/chjj/marked/issues/689). 

 Comments: 
 Comment 0: this is an upstream issue. There's been virtually no follow up since this has been logged either. Closing.
{'source': 'GithubIssues', 'repo_name_with_owner': 'elastic/kibana', 'issue_id': 171972715, 'repo_stars': 17982.0}