Add separate wrapper for #'s in headers

[dcoales]

Thanks very much for wrapping headers in bullets so that I can style them. So that I can style it to look nicer in edit mode would it be possible to split the #'s from the text like the standard renderer does ? The #'s are usually wrapped in "cm-formatting", "cm-formatting-header" and "cm-formatting-header-n" classes and the text in the "cm-header" and "cm-header-n". That way I can style the cm-formatting-header class to hide the #'s in edit mode until you click on the line.

[dcoales]

I've made an attempt at adding this functionality and submitted a pull request. I appear to have checked in package-lock.json. I don't know if that's a problem - I'm a complete noob at this, sorry.