code tag should have language as class

Many themes are customized by language and depend on the `code` element having the language as an additional class, i.e:

`<code class="hljs javascript">...</code>`

Currently there's no way to lookup a language name by alias, so until [#359](https://github.com/isagalaev/highlight.js/pull/359) is released this still won't work if the user supplies an alias.