Enhance link element to support HTTP(S) URLs in xl:href attribute

[fbolton]

In DocBook 5, it is possible (in fact, recommended) to define ordinary HTTP hyperlinks using the following syntax:

<link xl:href="http://foo">Optional Text for Link</link>

Where xl references the xlink Namespace URI. This can be rendered easily enough in AsciiDoc as:

http://foo[Optional Text for Link]