Feat request: Please allow us to have access to transport config

[tgrushka]

gomitmproxy.NewProxy starts out as follows:

// NewProxy creates a new instance of the Proxy
func NewProxy(config Config) *Proxy {
	proxy := &Proxy{
		Config: config,
		transport: &http.Transport{
			// This forces http.Transport to not upgrade requests to HTTP/2
			// TODO: Remove when HTTP/2 can be supported
			TLSNextProto:          make(map[string]func(string, *tls.Conn) http.RoundTripper),
			Proxy:                 http.ProxyFromEnvironment,
...

The Proxy: http.ProxyFromEnvironment is what affects me specifically. I am using an upstream proxy, and having to os.Setenv... to make it work is very inelegant:

os.Setenv("ALL_PROXY", upstreamProxy)
os.Setenv("HTTP_PROXY", upstreamProxy)
os.Setenv("HTTPS_PROXY", upstreamProxy)
fmt.Println("Upstream Proxy =", upstreamProxy)

Being able to pass this in, perhaps via the Config would be amazing. I would be happy to make a PR, please suggest if can be added to Config (maybe simple/non-breaking) or should be done some other way.

Thank you.

[ameshkov]

Makes sense, thank you!

I'd suggest doing it the following way:

1. Add an optional ProxyDialer proxy.Dialer field to Config.
2. When creating the transport check use it OR use http.ProxyFromEnvironment if it's nil. Please explain this logic in the field comment.