Prefer cmark for mdvimb

On Fedora, commonmark is from nodejs-commonmark, which is long-term
broken.

dotfiles/bash/main.bashrc

 export PRETTY=color
 export PRETTY_DEBUG_EXCLUDE=inigrep
 
-export MDVIMB__CONVERTER=commonmark
+export MDVIMB__CONVERTER=cmark
 export MDVIMB__STYLE=darkzen    # pergamen
 
 #shellcheck disable=SC2034