Support GFM checklists

[ethomson]

Checklist support (like offered in GFM) would be amazing: https://github.com/blog/1375-task-lists-in-gfm-issues-pulls-comments

I realize that it could be a bit contentious to have checklists (why not use linked work items instead?) but it would be a nice way to have a lighter-weight process.

[Tom-Spring]

We would like checkboxes as well in order to check of DOD on each work item