GFM: be more clever on parsing fenced codeblock

[tsl0922]

This pr adds support for fenced code block with whitespace before the ```

source:

  ```js
  console.log("hello");
  ```

github result:

  console.log("hello");

kramdown result:

<p><code>js
  console.log("hello");
 </code></p>

[gettalong]

\s*? is not correct since after testing I found that only a maximum of three spaces is allowed by GFM on Github.

[tsl0922]

Yes, it can be more strictly here, changed to [ ]{,3}. More than 3 spaces will be identified as normal code block at github, so does kramdown.

[tsl0922]

[ ]{,3} failed travis test for 1.8.7, changed it to [ ]{0,3}

[gettalong]

Thanks for the changes - this looks good to me now! I will include it for the next release.

[gettalong]

Thanks for the pull request - will be in the next release.