Currently links entered using markdown formatting do not have the 
nofollow/noreferer relations. These relations are present to reduce the 
value of link spamming.

In msg7010 Cedric indicates there is an option to do this in markdown2
but not mistune or markdown.

It looks like both mistune and markdown support plugins. So we could
write plugins that can add this attribute and initialize the markdown
interpreters to use the plugin.

I think this should always be enabled just like normal text format.
Note that it is enabled only for non-self-referring url's. So issue234
msg20 etc. links do not have the anti-spam relations.