Support for automatically detecting system proxy settings

Hello Louis,

Opening this issue as requested by you on Discord.

On Unix-like systems, variables like `all_proxy`, `http_proxy`, `https_proxy`, `no_proxy` are often automatically detected by tools (e.g. `curl`). This is more of a convention than a standard, nevertheless it's very common.

Some additional info:
https://superuser.com/questions/944958/are-http-proxy-https-proxy-and-no-proxy-environment-variables-standard

This would enable `httpc` to be used from behind a proxy (which is sometimes enforced, e.g. company networks). Additionally, it would be useful to be able to override the proxy in code on a per request basis.

Thanks,
David