issue2551098: markdown links missing rel="noreferer nofollow" Links generated by all markdown backends are missing the noopener and nofollow relation that roundup's normal text -> html core adds to prevent security issues and link spam. Now rel="nofollow" is added to links generated by markdown2 backends and rel="nofollow noopener" for mistune and markdown backends. Markdown2 isn't as programable as the other two backends so I used the built-in nofollow support. This means that a user that generates a link that opens in a new window can manpulate the parent window.