Use HTML5 recommended style of language on code blocks

See #108. As @shurcooL suggested, I just yanked the github code block function straight out, but I'm indifferent to whether or not it's removed, so any other opinions are welcome.

Not sure what constitutes a breaking change in this repository, so if this calls for a new release tag, I'll leave that up to the maintainers.
If you're doing that, there's no point in keeping `BlockCodeNormal()` as a separate function, is there. Its contents can just be moved directly into here, leaving just `BlockCode()`.