are you building this out hoopy frood?
I would prefer the == syntax since !! sometimes gets confused with images.
I think you should follow how the https://stackedit.io/app# app handles highlighting
Example below
It should also have a binding key like when you press CTRL+B it automatically gives you ****. Instead have it binded on either
- CTRL+H
- CTRL+SHIFT+H
to trigger ==== or around whatever is selected