Checkboxes `[ ]` stay are not prepended with hyphen `- [ ]`

[hsandt]

Checkboxes [ ] stay are not prepended with hyphen - [ ] (same for variants [x], etc.), so they just appear as normal [ ] characters in standard Markdown editors.

In comparison, native Markdown (pandoc) export adds hyphens.

[ ] TODO
[*] DONE
[x] FAILED

Zim to Markdown gives:

[ ] TODO
[*] DONE
[x] FAILED

Expected:

- [ ] TODO
- [*] DONE
- [x] FAILED