Editing markdown source

[pico40]

Is your feature request related to a problem? Please describe.
I like to manipulate markdown code instead of just seeing the rendered result in a WYSIWYG editor.

Describe the solution you'd like
I would like to have a button or tabs (like on github) to switch between a source editor and a WYSIWYG editor.

Describe alternatives you've considered
An alternative would be two have two completely different editors.

[juliusknorr]

Duplicate of #123

#123 (comment)