add HTML_NOFOLLOW_LINKS

If one of the target uses of blackfriday is formatting and displaying untrusted user data, I think having the ability to force rel="nofollow" into the links is probably important for spam reduction purposes:

https://support.google.com/webmasters/answer/96569?hl=en

http://www.searchenginejournal.com/all-wikipedia-links-are-now-nofollow/4288/