Markdown.pl: support ``` code blocks

Non-indented code blocks may be used by preceding them
with a line consisting of 3 (or more) ` characters and
following them with a line consisting of the same
number of backtick characters.

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

Markdown.pl

@@ -964,6 +964,29 @@ sub _DoCodeBlocks {
 	    $result;
 	}egmx;
 
+    $text =~ s{
+	    (?:\n|\A)
+		``(`+)\w*\n
+	    (		    # $1 = the code block -- one or more lines, starting with ```
+	      (?:
+		.*\n+
+	      )+?
+	    )
+	    (?:(?:``\1(?:\n|\Z))|\Z) # and ending with ``` or end of document
+	}{
+	    my $codeblock = $2;
+	    my $result; # return value
+
+	    $codeblock = _EncodeCode($codeblock);
+	    $codeblock = _Detab($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;
+	}egmx;
+
     return $text;
 }
 

basics.text

@@ -296,7 +296,11 @@ Output:
 
 To specify an entire block of pre-formatted code, indent every line of
 the block by 4 spaces or 1 tab. Just like with code spans, `&`, `<`,
-and `>` characters will be escaped automatically.
+and `>` characters will be escaped automatically. Alternatively an
+entire block of pre-formatted code may be preceded with a line consisting
+of 3 backtick quotes (or more) and followed by a line consisting of the
+same number of backtick quotes -- in which case the code itself does not
+need to be additionally indented.
 
 Markdown:
 

syntax.text

@@ -481,12 +481,24 @@ of a code block are interpreted literally. Markdown wraps a code block
 in both `<pre>` and `<code>` tags.
 
 To produce a code block in Markdown, simply indent every line of the
-block by at least 4 spaces or 1 tab. For example, given this input:
+block by at least 4 spaces or 1 tab. Alternatively precede the block with
+a line consisting of 3 backtick quotes (or more) and follow it with a
+line consisting of the same number of backtick quotes -- in this case the
+code lines themselves do not require any additional indentation.
+For example, given this input:
 
     This is a normal paragraph:
 
         This is a code block.
 
+Or this equivalent input:
+
+    This is a normal paragraph.
+
+    ```
+    This is a code block.
+    ```
+
 Markdown will generate:
 
     <p>This is a normal paragraph:</p>
@@ -494,8 +506,11 @@ Markdown will generate:
     <pre><code>This is a code block.
     </code></pre>
 
-One level of indentation -- 4 spaces or 1 tab -- is removed from each
-line of the code block. For example, this:
+Note that when using the 3 backtick quotes technique, the blank line
+before the start of the code block is optional. One level of
+indentation -- 4 spaces or 1 tab -- is removed from each
+line of the code block unless the 3 backtick quotes are used.
+For example, this:
 
     Here is an example of AppleScript:
 
@@ -513,7 +528,13 @@ will turn into:
     </code></pre>
 
 A code block continues until it reaches a line that is not indented
-(or the end of the article).
+(or the end of the article) when using the indentation technique or
+until a line consisting of the same number of backtick quotes is found
+when using the 3 backtick quotes technique.
+
+Note that the 3 backtick quotes (or more) must appear at the beginning
+of the line.  To include a code block within a list (or other indented
+element), the indentation technique must be used.
 
 Within a code block, ampersands (`&`) and angle brackets (`<` and `>`)
 are automatically converted into HTML entities. This makes it very