I am working on a wysiwg editor plugin at the moment. Please read about the details in this thread:
you can also write test posts on
although this link might change/break later ( its my personal dev instance until my site is launched).
It works already quite well, but certain discourse additions to markdown are still only visible in the preview until I switch over to html only posts. But I recon thats okay for beginners as they wont use these features anyway.