Option to make URLs visible in printed PDF

[fwilhe]

Links in an Asciidoc become links in the PDF. That's fine for screen reading, but not so much for printing. I would like to have an option to specify that links are displayed in a "reference notation", like

this is important [1], and so on

1: http://foo.bar

[mojavelinux]

This is a duplicate of #463, which is now resolve. The details about how to use it are documented in that issue.