Add support for tasks/checkboxes in pull request descriptions

Having tasks (or checkboxes) in comments makes PR reviews a little easier and helps other reviewers see the status of a PR by the number of tasks left unticked. It would be useful to create a PR with a task list by default (e.g. A PR that is waiting on another one to go before it can be implemented might have a checkbox saying so in the description) by allowing checkboxes to be added to the description when the PR is created.

It would be massively useful for my workflow where I inject a pull request template into the description (using refined bitbucket). The pull request template by default has tasks (as markdown checkboxes) that need to be ticked off.

If those checkboxes were tracked like tasks it would be massively useful.