Support http_proxy and https_proxy environment variables

[gmile]

It would be great if Req supported reading http_proxy and https_proxy environment variables out of the box. A use case I have in mind is being able to debug HTTP interaction in programs that use Req as an HTTP client. For example, via tools like mitmproxy.

In my own programs, I can use proxy settings introduced in https://github.com/wojtekmach/req/pull/149 or even go as far as to use System.get_env to optionally read http_proxy and https_proxy ENV vars. However, for packages that rely on Req this may be much harder and requi modification of the package's source code (unless I am missing something, of course).

P.S. I am not sure if this is the right place to file an issue, or if it should be Finch 🙈 Mint issue tracker would not the right place, it seems (based on https://github.com/elixir-mint/mint/issues/92)