Add HTTP proxy functionality

[k3d3]

Add functionality that lets Panamax run through a web proxy. Hopefully this shouldn't be too difficult, as reqwest is heavily used and has built-in proxy functionality.

The proxy should be settable via the HTTP_PROXY environment variable, or an optional http_proxy option in mirror.toml, under the [mirror] section.

[k3d3]

Never mind. It appears Reqwest uses system proxies by default anyways, which means HTTP_PROXY and HTTPS_PROXY environment variables will be respected.