Markdown.pl: handle some limited [[wiki style links]]

The double bracket links are pervasive in source documents.

Recognize them and process them.  At this point only links
that reference an absolute URL are recognized and turned
into clickable links.

Everything else is passed on through unchanged.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>

Markdown.pl

@@ -730,6 +730,25 @@ sub _EscapeSpecialChars {
 }
 
 
+sub _ProcessWikiLink {
+    my ($link_text, $link_loc) = @_;
+    if (defined($link_loc) && $link_loc =~ m{^(?:http|ftp)s?://\S+$}i) {
+	# Just rewrite it to [...](...) form
+	return "[".$link_text."](".$link_loc.")";
+    }
+    if (defined($link_loc)) {
+	# We don't handle any other kind of "bar" links yet
+	return undef;
+    }
+    if ($link_text =~ m{^(?:http|ftp)s?://\S+$}i) {
+	# Just rewrite it to [...](...) form
+	return "[".$link_text."](".$link_text.")";
+    }
+    # We don't handle any other wiki-style links yet
+    return undef;
+}
+
+
 sub _DoAnchors {
 #
 # Turn Markdown link shortcuts into XHTML <a> tags.
@@ -737,7 +756,32 @@ sub _DoAnchors {
     my $text = shift;
 
     #
-    # First, handle reference-style links: [link text] [id]
+    # First, handle wiki-style links: [[wiki style link]]
+    #
+    $text =~ s{
+	(		    # wrap whole match in $1
+	  \[\[
+	    ($g_nested_brackets) # link text and id = $2
+	  \]\]
+	)
+    }{
+	my $result;
+	my $whole_match = $1;
+	my $link_text	= $2;
+	my $link_loc    = undef;
+
+	if ($link_text =~ /^(.*)\|(.*)$/s) {
+	    $link_text = $1;
+	    $link_loc = $2;
+	}
+
+	$result = _ProcessWikiLink($link_text, $link_loc);
+	defined($result) or $result = $whole_match;
+	$result;
+    }xsge;
+
+    #
+    # Next, handle reference-style links: [link text] [id]
     #
     $text =~ s{
 	(		    # wrap whole match in $1
@@ -783,7 +827,7 @@ sub _DoAnchors {
     }xsge;
 
     #
-    # Next, inline-style links: [link text](url "optional title")
+    # Finally, inline-style links: [link text](url "optional title")
     #
     $text =~ s{
 	(		# wrap whole match in $1