Support HTTP_PROXY env

[imba-tjd]

Is your feature request related to a problem? Please describe.
It's conventional to support HTTP_PROXY (and HTTPS_PROXY) environment variables to configure proxy for web requests. As I tested this CLI app doesn't support it when using py install.

[zooba]

Good idea, though it will use your default proxy settings already. Adding support to override those wouldn't necessarily be a bad thing, though.