Adds support for github style triple backtick code blocks

pkgs/markdown/lib/src/markdown/block_parser.dart

diff --git a/pkgs/markdown/lib/src/markdown/block_parser.dart b/pkgs/markdown/lib/src/markdown/block_parser.dart
index 145743c..e1d04ec 100644
--- a/pkgs/markdown/lib/src/markdown/block_parser.dart
+++ b/pkgs/markdown/lib/src/markdown/block_parser.dart
@@ -19,6 +19,9 @@
 /// A line indented four spaces. Used for code blocks and lists.
 final _RE_INDENT = new RegExp(r'^(?:    |\t)(.*)$');
 
+/// GitHub style triple quoted code block.
+final _RE_CODE = new RegExp(r'^```(\w*)$');
+
 /// Three or more hyphens, asterisks or underscores by themselves. Note that
 /// a line like `----` is valid as both HR and SETEXT. In case of a tie,
 /// SETEXT should win.
@@ -94,6 +97,7 @@
           new SetextHeaderSyntax(),
           new HeaderSyntax(),
           new CodeBlockSyntax(),
+          new GitHubCodeBlockSyntax(),
           new BlockquoteSyntax(),
           new HorizontalRuleSyntax(),
           new UnorderedListSyntax(),
@@ -234,7 +238,44 @@
     childLines.add('');
 
     // Escape the code.
-    final escaped = Strings.join(childLines, '\n');//classifySource();
+    final escaped = childLines.join('n');//classifySource();
+
+    return new Element.text('pre', escaped);
+  }
+}
+
+/// Parses preformatted code blocks between two ``` sequences.
+class GitHubCodeBlockSyntax extends BlockSyntax {
+  RegExp get pattern => _RE_CODE;
+
+  List<String> parseChildLines(BlockParser parser) {
+    final childLines = <String>[];
+    parser.advance();
+    while (!parser.isDone) {
+      var match = pattern.firstMatch(parser.current);
+      if (match == null) {
+        childLines.add(parser.current);
+        parser.advance();
+      } else {
+        parser.advance();
+        break;
+      }
+    }
+    return childLines;
+  }
+
+  Node parse(BlockParser parser) {
+    // Get the syntax identifier, if there is one.
+    var syntax = pattern.firstMatch(parser.current).group(1);
+    print('syntax: $syntax');
+    
+    final childLines = parseChildLines(parser);
+
+    // The Markdown tests expect a trailing newline.
+    childLines.add('');
+
+    // Escape the code.
+    final escaped = childLines.join('\n').trim();
 
     return new Element.text('pre', escaped);
   }