config: add Dialer option to connect through a proxy

[HaoweiCh]

import "golang.org/x/net/proxy"

if dialer, err := proxy.SOCKS5("tcp", "192.168.3.1:23456", nil, nil); err != nil {
    log.Fatal(err)
} else {
    cfg..Dialer = dialer
}

[gknw]

I want to vote for accept this PR. We are looking for a way to manage NSQ connection, and a custom dialer solved (partially) our problem. It's a convenient thing.