If a language-xxx or lang-xxx class is already set, don't overwrite

[ldionne]

Right now, we set the language-xxx class of the <code> element based on the extension of the file. In some cases, it is useful to specify the language explicitly instead of relying on the extension of the file. If a language-xxx or lang-xxx class is already set on the element, we should leave it and not add our own.