switching Heading type should replace current header

[Matthijs Brouwer]

First of all: Great plugin! Thanks!
Second of all: If I can make some time I'll see if I can resolve this myself and post a pull-request.

The issue: When I use a heading and then press the other heading button, the # marks are not replaced but added, so I get something like this:

## ### Heading switched from h2 to h3

[Jonas Döbertin]

Sounds reasonable. Great idea, I'll implement this as soon as possible!

Thanks 👍

[Jonas Döbertin]

This has been added to the develop branch.

[Matthijs Brouwer]

thanks! I'll give it a go soon :-)