fix fenced code blocks for markdown

Author: holman

vim/vim.symlink/syntax/markdown.vim

@@ -67,6 +67,7 @@ syntax match  mkdCode      /^\s*\n\(\(\s\{4,}[^ ]\|\t\+[^\t]\).*\n\)\+/
 syntax match  mkdLineBreak /  \+$/
 syntax region mkdCode       start=/\\\@<!`/     end=/\\\@<!`/
 syntax region mkdCode       start=/\s*``[^`]*/  end=/[^`]*``\s*/
+syntax region mkdCode       start=/\s*```[^`]*/ end=/[^`]*```\s*/
 syntax region mkdBlockquote start=/^\s*>/       end=/$/           contains=mkdLineBreak,mkdLineContinue,@Spell
 syntax region mkdCode       start="<pre[^>]*>"  end="</pre>"
 syntax region mkdCode       start="<code[^>]*>" end="</code>"