footer: enable html/markdown formatting

It would be very useful to be able to insert hyperlinks in the footer (for example with the link to the data source), as well as having a word in bold, to highlight it.

If markdown could be enabled, that would be perfect.

```
    {
      type: "footer",
      text: `Lorem **ipsum**
[2023](https://example.com)`
    }
```

Thank you