Plugin support.

  There is now the facility to load plugins dynamically within sites.
 These are used *solely* for expanding variables.

  Currently we support:

    name: value
    name: file_glob( pattern )
    name: read_file( filename )

  With plugins we can now support:

    name: call_plugin foo arg1 arg2 .. argN

  This allows interesting things, though it needs to be documented.

  Plugins receive a reference to:

* The site object.
* The page object, the plugin was called from.
* The arguments supplied, if any.