Define external Dial function to allow next hop Proxy for https connection

[huaraz]

I see in proxy.go that outbound connections are made using local proxy environment variable. This does not work for https:// connection.

    prx := &Proxy{          
            Rt: &http.Transport{TLSClientConfig: &tls.Config{},
                    Proxy: http.ProxyFromEnvironment},
            MitmChunked: true,
            signer:      NewCaSignerCache(1024),
    }

To use the proxy for TLS connection something like https://github.com/wrouesnel/go.connect-proxy-scheme is required which can be defined in the Proxy structure and overwritten from default net.Dial.

This would then be used in context.go

    case ConnectProxy:
            conn, err := **ctx.Prx.Dial**("tcp", host)
            if err != nil {

and also MITM outbound connection.

Would that make sense ?