Make IP address header configurable

[flyte]

Sometimes the X-Forwarded-For header isn't the one that contains the real user's IP address. For example, if you're behind a CloudFlare proxy, the header you want to use is CF-Connecting-IP.

It would be useful if this app had a setting that could specify which request.META key it used; defaulting to HTTP_X_FORWARDED_FOR as it's set up with now.