Add reverse-proxy support consistent with httpHosts

If a reverse proxy is in front of ProcessWire, *$config->httpHost* is not getting set to the forwarded host, and as a result surfacing *Unrecognized HTTP host* errors and/or not generating the appropriate URL for "veiw" links in the admin. This request adds optional support to address this. Since HTTP_X_FORWARDED_HOST can be spoofed, the feature is disabled by default.

I couldn't find a reasonable way to add support without editing core, but I'm open to non-core approaches if it's possible.