Markdown.pl: handle non-backticks-delimited code blocks properly

The regular, indented-by-four-spaces, code blocks do not nest nor
should they.  But they were nesting if they were located inside a
list.  Fix this by hashifying them and not unhashifying them until
the very end.

Also there's a kludge in the code that says:

  # Turn double returns into triple returns, so that we can make a
  # paragraph for the last item in a list, if necessary

Unfortunately that perverts blank lines inside a code block.
Fix this by changing the perversion so that it accomplishes the
same thing but has an exact inverse and apply that inverse before
formatting code blocks.

Code blocks inside lists should now format correctly (and this
does fix the example in the README that was previously formatted
incorrectly).

Finally, a code block at the very beginning of the file preceded
by a single blank line would not have been recognized (but if it
were preceded by none or two or more it would have).  Now it will
be recognized properly.

And one more thing.  Since we're in there tweaking code blocks,
wrap the output in a <div>...</div> section and insert a null
<dl></dl> right after the opening <div> tag.  This makes sure
the displayed code block will not end up getting mashed up
against something it shouldn't be mashed up against.

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

Markdown.pl

@@ -66,6 +66,7 @@ my %g_urls;
 my %g_titles;
 my %g_block_ids;
 my %g_html_blocks;
+my %g_code_blocks;
 my %opt;
 
 # Return a "block id" to use to identify the block that does not contain
@@ -344,6 +345,7 @@ sub Markdown {
     %g_titles = ();
     %g_block_ids = ();
     %g_html_blocks = ();
+    %g_code_blocks = ();
     $g_list_level = 0;
 
     # Standardize line endings:
@@ -373,6 +375,9 @@ sub Markdown {
 
     $text = _RunBlockGamut($text);
 
+    # Unhashify code blocks
+    $text =~ s/(\005\d+\006)/$g_code_blocks{$1}/g;
+
     $text = _UnescapeSpecialChars($text);
 
     return $text . "\n";
@@ -402,7 +407,7 @@ sub _HashBTCodeBlocks {
 	    $codeblock =~ s/\A\n+//; # trim leading newlines
 	    $codeblock =~ s/\s+\z//; # trim trailing whitespace
 	    $codeblock = _EncodeCode($codeblock); # or run highlighter here
-	    $codeblock = "<pre><code>" . $codeblock . "\n</code></pre>";
+	    $codeblock = "<div><dl></dl><pre><code>" . $codeblock . "\n</code></pre></div>";
 
 	    my $key = block_id($codeblock);
 	    $g_html_blocks{$key} = $codeblock;
@@ -980,7 +985,7 @@ sub _DoLists {
 		my $list_type = ($3 =~ m/$marker_ul/) ? "ul" : "ol";
 		# Turn double returns into triple returns, so that we can make a
 		# paragraph for the last item in a list, if necessary:
-		$list =~ s/\n{2,}/\n\n\n/g;
+		$list =~ s/\n\n/\n\n\n/g;
 		my $result = _ProcessListItems($list, $marker_any);
 		$result = "<$list_type>\n" . $result . "</$list_type>\n";
 		$result;
@@ -1006,7 +1011,7 @@ sub _DoLists {
 		my $list_type = ($3 =~ m/$marker_ul/) ? "ul" : "ol";
 		# Turn double returns into triple returns, so that we can make a
 		# paragraph for the last item in a list, if necessary:
-		$list =~ s/\n{2,}/\n\n\n/g;
+		$list =~ s/\n\n/\n\n\n/g;
 		my $result = _ProcessListItems($list, $marker_any);
 		$result = "<$list_type>\n" . $result . "</$list_type>\n";
 		$result;
@@ -1094,7 +1099,7 @@ sub _DoCodeBlocks {
     my $text = shift;
 
     $text =~ s{
-	    (?:\n\n|\A)
+	    (?:\n\n|\A\n?)
 	    (		# $1 = the code block -- one or more lines, starting with indent_width spaces
 	      (?:
 		(?:[ ]{$opt{indent_width}})  # Lines must start with indent_width of spaces
@@ -1104,15 +1109,16 @@ sub _DoCodeBlocks {
 	    ((?=^[ ]{0,$opt{indent_width}}\S)|\Z) # Lookahead for non-space at line-start, or end of doc
 	}{
 	    my $codeblock = $1;
-	    my $result; # return value
 
+	    $codeblock =~ s/\n\n\n/\n\n/g; # undo "paragraph for last list item" change
 	    $codeblock = _EncodeCode(_Outdent($codeblock));
 	    $codeblock =~ s/\A\n+//; # trim leading newlines
 	    $codeblock =~ s/\s+\z//; # trim trailing whitespace
 
-	    $result = "\n\n<pre><code>" . $codeblock . "\n</code></pre>\n\n";
-
-	    $result;
+	    my $result = "<div><dl></dl><pre><code>" . $codeblock . "\n</code></pre></div>";
+	    my $key = block_id($result);
+	    $g_code_blocks{$key} = $result;
+	    "\n\n" . $key . "\n\n";
 	}egmx;
 
     return $text;
@@ -1232,15 +1238,6 @@ sub _DoBlockQuotes {
 	    $bq = _RunBlockGamut($bq);	 # recurse
 
 	    $bq =~ s/^/  /mg;
-	    # These leading spaces screw with <pre> content, so we need to fix that:
-	    $bq =~ s{
-		    (\s*)(<pre>.+?</pre>)
-		}{
-		    my ($indent, $pre) = ($1, $2);
-		    $pre =~ s/^  //mg;
-		    $indent.$pre;
-		}egsx;
-
 	    "<blockquote>\n$bq\n</blockquote>\n\n";
 	}egmx;
 
@@ -1266,7 +1263,7 @@ sub _FormParagraphs {
     # Wrap <p> tags.
     #
     foreach (@grafs) {
-	unless (defined( $g_html_blocks{$_} )) {
+	unless (defined($g_html_blocks{$_}) || defined($g_code_blocks{$_})) {
 	    $_ = _RunSpanGamut($_);
 	    s/^([ \t]*)/<p>/;
 	    $_ .= "</p>";