Request: Add checkboxes feature

[chyde]

In my note taking, I use a lot of checkboxes, and it would be nice if this existed in your package!

Github implements them as
- [x] Finish my changes

• Finish my changes

However, I usually write them as such
[X] remove unnecessary console logs

Either way, might be nice. Only issue I foresee is that you use square braces for language definitions, but 'X' and ' ' are not languages (yet!)