Render <pre><code> constructs as fenced code blocks

[martin-martin]

Markdown that uses backticks to indicate code blocks instead of indentation has the advantage that it is very easy to add language-identifier information to it, e.g.:

```python
print("hello")
```

Is there an option to convert <pre><code> HTML into fenced code blocks, or would you be interested to add it?

[martin-martin]

Wrote a possible adaptation for this in #13

Hope this is helpful and makes sense, happy for any feedback and comments