Add option to set value of RemoteAddr/RemoteIP by middleware

[lafriks]

Maybe pointer to net.Addr could be added to RequestCtx set that it could be set by middleware that if set would be used by RequestCtx.RemoteAddr() instead of one from c.

This would allow implementing more robust and generic middlewares for handling X-Real-IP and/or X-Forwarded-For etc

[erikdubbelboer]

A pull request is welcome. I'm not sure if a pointer to a net.Addr or a method to set it would be better.

[lafriks]

I can create a PR if idea is acceptable. I meant adding private variable to struct RequestCtx and adding method something like SetRemoteAddr that would set it.

[lafriks]

@erikdubbelboer PR submitted