Make fenced code blocks add the `class` to `code` rather than `pre`

This matches the convention set out in the HTML Living Standard (http://www.whatwg.org/specs/web-apps/current-work/multipage/semantics.html#the-code-element) and makes existing syntax highlighting scripts recognize the programming language used.

Ref. http://talk.standardmarkdown.com/t/fenced-code-blocks-should-add-class-to-code-rather-than-pre-matching-the-html-best-practice/136