Markdown.pl: remove control characters first

Before doing any other processing, eliminate any control
characters that might be present.

There should not be any, but just in case.

The only "control" characters kept are ASCII codes
9, 10, 12 and 13 (tab, nl, ff, cr).

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

Markdown.pl

@@ -404,6 +404,10 @@ sub Markdown {
     my $_text = shift;
     defined $_text or $_text='';
 
+    {
+	use bytes;
+	$_text =~ s/[\x00-\x08\x0B\x0E-\x1F\x7F]+//gso;
+    }
     my $text;
     if (Encode::is_utf8($_text) || utf8::decode($_text)) {
 	$text = $_text;