fix(parser): code block language class

previously the code block had a class with the name found in the markdown code notation:

```js
console.log(test);
```
<pre>
 <code class=js>
console.log(test);
 </code>
</pre>

now the parser generates the output according to https://html.spec.whatwg.org/multipage/text-level-semantics.html#the-code-element

<pre>
 <code class=language-js>
console.log(test);
 </code>
</pre>

parser/tags.go

@@ -136,7 +136,7 @@ type CodeBlock struct {
 }
 
 func (p CodeBlock) String() string {
-	return fmt.Sprintf("<pre class=\"%s\"><code>%s</code></pre>", p.language, p.text)
+	return fmt.Sprintf("<pre class=\"language-%s\"><code>%s</code></pre>", p.language, p.text)
 }
 
 // <code></code>, inline code element, contains plain text