bug: unable to remove header formatting in a specific scenario

**Is your feature request related to a problem? Please describe.**
In the markdown editor toolbar, there is no easy way to remove heading level stylings once applied. From what I can tell, the only way to remove a heading level styling from text is to switch to the raw markdown mode and remove the `#` formatting there.

**Describe the solution you'd like**
Add a toolbar option to remove a heading styling if applied to selected text. I can thing of 3 ways this could be implemented.

1. When a heading level option in the dropdown is applied/active, clicking the dropdown item again toggles the formatting off.
2. Add a new heading dropdown option "Remove"
3. Add a new toolbar button that more generally removes any applied markdown formatting from a selection.