Use HTML5 recommended style of language on code blocks

For code blocks that contain a certain language of code, the recommended
attribute structure is <pre><code class="language-foo">. This also
corresponds to the behavior expected by various JS syntax highlighters.

The GitHub code block implementation was obsolete, and identical to the
normal implementation except for its attribute structure, so it was
removed.

Closes #108.
Merge pull request #109 from tummychow/html5-code-block

Use HTML5 recommended style of language on code blocks