Implement basic "full" proxy support

In this mode, it will act as a proxy, connecting to any website that is
requested from it.  The original `-proxy-url` flag is more like a
"fronting" server, whereas this new `-proxy` option is a generic proxy.

go/fake-http/fake-http.go

@@ -29,6 +29,7 @@ var flags struct {
	proxyClientKey  string
	proxyMinikube bool
	proxy         bool
	cache         bool
}
@@ -39,6 +40,7 @@ func init() {
	flag.StringVar(&flags.proxyClientKey, "proxy-client-key", "", "Client key to use when connecting to proxy")
	flag.BoolVar(&flags.proxyMinikube, "proxy-minikube", false, "Shortcut for -proxy-url https://$(minikube ip):8443 -proxy-client-cert ~/.minikube/client.crt -proxy-client-key ~/.minikube/client.key")
	flag.BoolVar(&flags.proxy, "proxy", false, "Proxy requests to any website")
	flag.BoolVar(&flags.cache, "cache", false, "Cache all requests")
}
@@ -75,8 +77,12 @@ func main() {
		stub := responses.Match(req)
		haveCachedStub := flags.cache && stub != nil
		var resp *http.Response
		if flags.proxyURL != "" && !haveCachedStub {
			resp = respondWithProxy(flags.proxyURL, &cert, w, req)
		if (flags.proxy || flags.proxyURL != "") && !haveCachedStub {
			proxyURL := flags.proxyURL
			if flags.proxy {
				proxyURL = req.RequestURI
			}
			resp = respondWithProxy(proxyURL, &cert, w, req)
		} else {
			resp = respondWithStub(stub, w, req)
		}