add proxy support

[abzcoding]

we can have socks5 proxy support

usage:

hget -proxy "127.0.0.1:12345" https://releases.ubuntu.com/20.04.1/ubuntu-20.04.1-desktop-amd64.iso

we can also use the env and forget about the flag, something like this:

proxyServer, isSet := os.LookupEnv("HTTP_PROXY")
if isSet {
    proxyUrl, err := url.Parse(proxyServer)
    if err == nil {
                dialer, err = proxy.FromURL(proxyUrl, proxy.Direct)
    }else if strings.HasPrefix("socks5://"){
                socksProxy := strings.Replace(proxyUrl, "socks5://", "", -1)
                dialer, err := proxy.SOCKS5("tcp", socksProxy, nil, proxy.Direct)
        }
}

let me know if you prefer the second method, so i can update the code accordingly