Add support for checkboxes made in markdown.
When moving from github which happily accepts checkboxes with
- [x] (in their checked state) and
- [ ] in their unchecked state allowing the user to lazily click on them when an bug or part of an issue opened is fixed. However on bitbucket those are not supported and are processed to:
- [ ] example that should become an checkbox for this test here but is not.
Otherwise the markdown on bitbucket is about to par with github's except for the checkboxes and stuff.