Add Content-Negotiation to actix-files

For the Semantic Web/Linked Data community, content negotiation is an important feature.

We could implement it similar to [how it's described here](https://pieterheyvaert.com/blog/2019/02/25/nginx-conneg/), where requesting `http://localhost:8080/test` with `Accept: text/turtle` then makes the server look for `test.ttl` on disk.

This could be enabled or disabled in the actix-files service.