Markdown.pl: be more flexible parsing backticks-delimited code blocks

Allow leading spaces before the backticks delimiters on the starting
and ending lines (up to one less than the indent width).

Then remove upto that number of leading spaces (based on the starting
backticks delimiter line) from each of the lines in the code block
itself.

This better matches how lax some other formatters are with backticks-
delimited code blocks parsing.

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

Markdown.pl

@@ -489,23 +489,27 @@ sub _HashBTCodeBlocks {
 #   Process Markdown backticks (```) delimited code blocks
 #
     my $text = shift;
+    my $less_than_indent = $opt{indent_width} - 1;
 
     $text =~ s{
 	    (?:(?<=\n)|\A)
-		``(`+)[ \t]*(?:([\w.+-]+)[ \t]*)?\n
-	     ( # $3 = the code block -- one or more lines, starting with ```
+		([ ]{0,$less_than_indent})``(`+)[ \t]*(?:([\w.+-]+)[ \t]*)?\n
+	     ( # $4 = the code block -- one or more lines, starting with ```
 	      (?:
 		.*\n+
 	      )+?
 	     )
-	    (?:(?:``\1[ \t]*(?:\n|\Z))|\Z) # and ending with ``` or end of document
+	    # and ending with ``` or end of document
+	    (?:(?:[ ]{0,$less_than_indent}``\2[ \t]*(?:\n|\Z))|\Z)
 	}{
 	    # $2 contains syntax highlighting to use if defined
-	    my $codeblock = $3;
+	    my $leadsp = length($1);
+	    my $codeblock = $4;
 	    $codeblock =~ s/[ \t]+$//mg; # trim trailing spaces on lines
 	    $codeblock = _Detab($codeblock, 8); # physical tab stops are always 8
 	    $codeblock =~ s/\A\n+//; # trim leading newlines
 	    $codeblock =~ s/\s+\z//; # trim trailing whitespace
+	    $codeblock =~ s/^ {1,$leadsp}//mg if $leadsp; # trim leading space(s)
 	    $codeblock = _EncodeCode($codeblock); # or run highlighter here
 	    $codeblock = "<div class=\"$opt{style_prefix}code-bt\"><pre style=\"display:none\"></pre><pre><code>"
 		. $codeblock . "\n</code></pre></div>";