[O] org-md fixed-width filter problems

[cberry]

The  :translate-alist for the md backend has no entry for fixed-width,
so it defaults to the parent org-e-html-fixed-width which uses

(format "<pre class=\"example\">\n%s</pre>" ...

So elements 

,----
| : like
| : this 
`----

get the html style markup.

Some of the markdown processors I use don't like this. pandoc ignores
the element entirely. The github blob generator copies it as verbatim.

I have been using (fixed-width . org-md-verbatim) instead, but I think
this is still not quite right. (IIUC, proper markdown uses four spaces
at bol for such elements while org-md-verbatim uses surrounding backtics.)

?

Chuck