Markdown.pl: recognize two ```-delimited blocks in a row

Backticks-delimited code blocks do not require a blank line
between them to be recognized (at least they're not supposed
to).

Recognize two such code blocks in a row by tweaking the
regex to use an assertion instead of an explicit match.

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

Markdown.pl

@@ -477,7 +477,7 @@ sub _HashBTCodeBlocks {
     my $text = shift;
 
     $text =~ s{
-	    (?:\n|\A)
+	    (?:(?<=\n)|\A)
 		``(`+)[ \t]*(?:([\w.+-]+)[ \t]*)?\n
 	     ( # $3 = the code block -- one or more lines, starting with ```
 	      (?: