go.blog/cmd/blog: serve Atom feed from old blog's feed URL
go.blog/cmd/blog: serve Atom feed from old blog's feed URL

R=golang-dev, minux.ma, rsc
CC=golang-dev
https://codereview.appspot.com/7833045

bryanxu/go-zh.blog,d0f/go-zh.blog,d0f/go-zh.blog,webfd/go-zh.blog,bryanxu/go-zh.blog,webfd/go-zh.blog