Added Javier Rojas' linktoimgonly.pm plugin.

From
  http://devnull.li/~jerojasro/files/linktoimgonly.pm
Updated linktoimgonly.pm to use htmllink().

Following Joey's suggestion in
  doc/forum/link_to_an_image_inside_the_wiki_without_inlining_it.mdwn

I also altered the directive syntax:
  [[!ltio img="image1.jpg"]]
is now
  [[!ltio image1.jpg]]
Documented the linktoimgonly plugin.
Fix WikiLinnk -> Wikilink typo in ltio.mdwn.