#3 Support for links

Links need to be added to the MiniDOC syntax. At
present they can only be embedded through HTML.

There should be 3 things that said syntax is capable
of. One is to make a link out of the marked text, using
the same text for the link caption as the HREF. The
second is to optionally specify it opens in a new
window. The third is to be able to set a custom caption
for the link tag.