Regex for language code block

Hello,

I often use language named code block, like so:

`````` markdown
    ```C
    int a;
    ```
``````

But your plugin doesn't support triple backtick.

A simple way to do that would be to add this line in code.vim:

`````` vim
    syntax region mkdCode    start=/\s*```[^`]*/   end=/[^`]*```\s*/
``````
