Hi Cedric:

in msg6983 you said:

> An alternative solution would be to provide a roundup
> configuration to define which markdown format to use.

Are you thinking of a config option:

  MarkdownSettings

where we can specify a set of key:value or array of string values
to be added to be merged into the extras or extensions dict/array passed 
to the markdown implementation?

Something similar could be done for mistune but it uses actual 
classes/objects in python to augment its plugin array. So interfaces.py 
would probably need to be used to override _import_mistune in some way.