Add support for proxy

[Plawn]

Is your feature request related to a problem? Please describe. I'm running behind a corporate proxy and crates such as reqwest automatically use the system proxy, maybe this crate could do the same or at least let the user specify a proxy

Describe the solution you'd like use hyper_proxy: https://docs.rs/hyper-proxy/latest/hyper_proxy/ to set the proxy for the http client

Thanks !