Adding menu item for easier access to markdown export

Hey there!

Thanks for your work so far, it's awesome. With my pull request, I am asking you to add a new feature to your script which puts a menu item to the document on load (it's a built-in hook point), allowing to trigger an export just with a click later in the process. If you like it, feel free to confirm the request and/or let me know if you have any questions.

Thanks,
Michael