Added Manoj Srivastava's org.pm plugin.

From
  http://www.golden-gryphon.com/blog/manoj/blog/2008/06/08/Using_org-mode_with_Ikiwiki/
on which page it lists the license as "GPL".
Update org.pm plugin for IkiWiki 3.00.

Also replace string postprocessing with org-export-... configuration.
Clean up default org-mode export options to only export the body section.
Decode UTF-8 when reading export results in org.pm.