Configure http proxy via command-line arg

[jpstotz]

For debugging purposes I was missing an option to redirect the traffic through an HTTP proxy.

I wasn't able to figure out how to properly throw an error when the specified proxy URL is not valid (I don't develop much in Go), so feel free point me to the correct way or modify my PR accordingly.

[majd]

Thanks for the PR. Can you test if it's enough to just set the DefaultTransport so that the HTTP_PROXY env variable is respected? https://pkg.go.dev/net/http#ProxyFromEnvironment

[jpstotz]

Setting HTTP_PROXY doesn't work, may be because all requests performed by ipatool are https urls. Therefore I tested HTTPS_PROXY and it works. Not sure why this had not worked when I tested it before making the code changes. I assume that means this PR is dispensable?

While looking at the network responses ipatool receives I noticed that ipatool ignores 99% of the meta data received and it only extracts specific data. Are there any plans to extract and display more meta data to the user, as IMHO this is the point where ipatool is unique. For downloading IPAs I can simply use the old iTunes desktop application, but for getting authenticated app meta data there is no alternative to my knowledge.