Remove hljs class from blocks of unhighlighted code

[valtlai]

The HTML spec recommends using a language-* class in the <code> element. Also, I don’t think there should be any class for a block of unhighlighted code.

(Original commit: aff30c3)

[oscarotero]

Well, it is not exactly a recomendation, only an example.

hljs class is used by highlight syntax highlighting library and it's also a good way to select only pre elements containing hightlighted code (it would be a bit harder with a random variable classname like language-*.

I think a better approach is allow to customize this by the user. Something like:

site.use(markdown({
  config: {
      highlight(code, language) {
        //Here your custom code.
      }
  }
});

In fact, it should work like that, but I just realized that it doesn't because default options are applied after user options, so they are always overrided (https://github.com/lumeland/lume/blob/master/plugins/markdown.js#L43). We should change that.

[valtlai]

Oh, that’s okay. Should we still remove the class from unhighlighted code?

[oscarotero]

Mmm, maybe yes...?
It would break some existing styles in docs, but it's easy to fix.

[valtlai]

I repurposed this PR to just remove the hljs class from the block of unhighlighted code.

[oscarotero]

Thanks!