Consider adding `rel="nofollow"` to external links coming from Markdown content

[wincent]

Split off from https://github.com/wincent/masochist/issues/17.

This is a feature we used to have in the old wikitext renderer (via the external_link_rel option, although I don't think I actually used it).

[wincent]

Example of a reason not to use it: https://en.wikipedia.org/wiki/Nofollow#Criticism_of_usage_by_Wikipedia