Committed in rev 6282:d30501bafdfb.

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 manipulate the parent window, not
great but monkey patching etc. is too fragile.

Markdown ads nofollow to all links including mailto etc. The other
backends add it only for http* type links.

Testing required a special mangler that converts markdown2 output
into the same output we get from markdown and mistune. This way
I have one valid test result for all three backends.