SI-8113 allow a newline between a link target and title

Parsing "[[http://foo.bar link title]]" stops at the first whitespace.
This breaks pretty badly in:

[[http://foo.bar
link title]]

It stops after "link", interprets what it parsed as a link to a member,
obviously fails, and then just ouputs "title".
It should at least return a proper error, or, even better, just allow a
newline between the target and title. I went for the latter.
Merge pull request #3960 from gourlaysama/wip/t8113

SI-8113 scaladoc: allow a newline between a link target and title