Added OnDial option which allows overriding dialing

[wangjia184]

This change allows to override dial.

For example, run http proxy over socks proxy

var (
	socksDialer proxy.Dialer = nil
)

func runHttpProxy(sock5Port, httpPort uint) error {

	var err error

	// create a socks5 dialer
	addr := fmt.Sprintf("127.0.0.1:%d", sock5Port)
	socksDialer, err = proxy.SOCKS5("tcp", addr, nil, proxy.Direct)
	if err != nil {
		return err
	}

	prx, _ := httpproxy.NewProxy()
	prx.OnDial = onHttpProxyDial

	addr = fmt.Sprintf(":%d", httpPort)
	return http.ListenAndServe(addr, prx)
}

func onHttpProxyDial(network, addr string) (conn net.Conn, err error) {
	return socksDialer.Dial(network, addr)
}