Should implement CORS handling

[benlabbe]

The handling of CORS request allows to deploy separately (different hosts) the backend and frontend servers of a joint web-application.

The CORS handling is a negotiation between a web-browser and the http-server. Some references below:

• https://fr.wikipedia.org/wiki/Cross-origin_resource_sharing
• https://enable-cors.org/server.html . this page refers especially a neat diagram to implement CORS handling : https://www.html5rocks.com/static/images/cors_server_flowchart.png

[benlabbe]

I posted an issue concerning CORS on the alternative qhttp library :
azadkuh/qhttp#53

Their developers might response with a standard implementation of CORS handeling that we could reproduce inside qhttpserver.

Otherwise, we should start to scratch our heads to implement it by our own.