cubicweb #15438011 [markdown] add support for non-standard (gitlab inspiered) code formating [open]

On gitlab issues when pasting some traceback or some code one does not have to indent the code, just surround it by

````
numerous lines of code
```

and it works out of the box. This avoids the need to copy/paste in an editor (emacs in my case) to indent by a few spaces the code for ReST or markdown.

prioritynormal
typeenhancement
done in<not specified>
closed by<not specified>