Markdown.pl: do not choke on \n inside attribute values

A tag such as this:

  <span style="
    lots: of;
    stuff: in;
    here: now;
  "></span>

Is perfectly valid.  Add the missing "s" pattern match
qualifier to make sure such attribute values do not end up
getting mangled.

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

Markdown.pl

@@ -3734,7 +3734,7 @@ sub _SanitizeAtt {
 
 sub _SanitizeAttValue {
     my $v = shift;
-    if ($v =~ /^([\042\047])(.*?)\1$/) {
+    if ($v =~ /^([\042\047])(.*?)\1$/s) {
 	return $1.escapeXML($2).$1;
     } else {
 	return '"'.escapeXML($v).'"';