go.blog/context: userip support for IPv6 host/port.
go.blog/context: userip support for IPv6 host/port.

Makes example usable on IPv6 systems eg where localhost is ::1.

Fixes golang/go#8486.

LGTM=adg
R=golang-codereviews, dave, adg
CC=golang-codereviews, sameer
https://golang.org/cl/117530044

jeffreyliguori/blog,gorakhargosh/blog,gorakhargosh/blog