Allow non-chunked transfer for larger files

[lolgesten]

Hi!

We're happy users of Rouille, however when sending larger files, we hit this issue:

https://github.com/tiny-http/tiny-http/blob/master/src/response.rs#L167-L168

    // if we don't have a Content-Length, or if the Content-Length is too big, using chunks writer
    let chunks_threshold = 32768;
    if entity_length.as_ref().map_or(true, |val| *val >= chunks_threshold) {
        return TransferEncoding::Chunked;
    }

This makes for a bad user experience, since they don't get any progress meter on downloads in the browser.

Would you consider a PR that lets us configure this behaviour? I.e. maybe raising the limit on a per-response basis?