New plugin argument 'show_deleted' determines whether "deleted" pages

New plugin argument 'show_deleted' determines whether "deleted" pages
are included in the listing.  Here, I'm refering to the "soft deleted"
pages --- where someone has just edited the page and erased all the
text.  "Hard deleted" pages, are, of course, gone for good and will
never be listed in RecentChanges...

The default value of 'show_deleted' is 'sometimes', which means deleted
pages will be shown iff 'show_minor' is true.

Also restored the rendering of links to deleted wiki-pages to using
linkUnknownWikiWord().

git-svn-id: svn://svn.code.sf.net/p/phpwiki/code/trunk@1654 96ab9672-09ca-45d6-a79d-3d69d39ca109