Markdown.pl: recognize adjacent standalone comments together

Combine adjacent (i.e. no separating blank line) standalone
XML comments into the same "block".

This is more efficient, better preserves the original comment
formatting and avoids an unfortunate side-effect that could
introduce unwanted extra paragraphs into the output.

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

Markdown.pl

@@ -827,6 +827,12 @@ sub _HashHTMLBlocks {
 			<!--
 			(?:[^-]|(?:-(?!-)))*
 			-->
+			(?:
+			    (?:[ \t]*\n[ \t]*)?
+			    <!--
+			    (?:[^-]|(?:-(?!-)))*
+			    -->
+			)*
 		    )
 		    [ ]*
 		    (?=\n{1,}|\Z)   # followed by end of line or end of document