Chunked http encoding

In one of our production system we have some larger css/js files to serve using quickconcat, because of embedded images/svg's. We see some verly late first byte responses for scripts served by quickconcat, caused by the fact that quickconcat buffers all content before sending the response. 

I would suggest using chunked transfer encoding in case the browser requested the files using http 1.1 or higher.

The server would send the quickconcat-enated files with one chunk per file to be combined.
This new approach would only affect requests which are not handled by the `HTTP_IF_MODIFIED_SINCE` header. 

I would love to send an PR for that, in case you accept..