Markdown.pl: add bdo to taga1p

The "bdo" (Bi-Directional Override) container element always requires
at least one attribute to be present for it to be valid.  Specifically,
in this case, the "dir" attribute.

Add "bdo" to the `%taga1p` (TAGs requiring Attributes count of 1 Plus)
hash to reflect this.  A bare "<bdo>" will now be passed through to
the output as "&lt;bdo>" when using the default options.

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

Markdown.pl

@@ -3118,7 +3118,7 @@ BEGIN {
 	compact coords height hspace ismap nohref noshade nowrap rowspan size
 	span shape valign vspace width
     ));
-    %taga1p = map({$_ => 1} qw(a area img map));
+    %taga1p = map({$_ => 1} qw(a area bdo img map));
 }