Add support for HTTP "Content negotiation"

I think it would be nice to support [content-negotiation](https://en.wikipedia.org/wiki/Content_negotiation) so a client can specify which format he wants via HTTP header `Accept` without having to modify the payload:
```
POST / HTTP/1.1
Host: orca
User-Agent: curl/7.61.0
Accept: image/svg+xml

$FIGURE_JSON_IN_BODY
```