Add option to not rewrite Host header in reverse-proxy

[tbroyer]

See #56 for the context. My downstream server doesn't look at X-Forwarded-Host, expecting the Host header to be the original one received by the reverse proxy (HAProxy in our case in production).

I understand that others proxying to a live server with virtual hosts won't want that behavior, so it should be an option, and preferably per-route.

Maybe add a notation like prefixing a URL with + (or another character) will preserve the req.Host instead of rewriting it to target.Host?