Add support for remote proxy usage

[tuxmea]

I planned to use npm-lazy-mirror in an internal network where all systems have to use a proxy for outbound connections.
The npm-lazy-mirror does not honor npm proxy settings (npm config) or http_proxy/https_proxy environment variables.

Either use the defaults (from npm or env variables) or allow a configuration section for proxy configuration.

[jdpaton]

To elaborate on this the search order for proxy settings is now:

1. CLI option flags --http_proxy, --https_proxy
2. Reading from npm config proxy/https-proxy
3. Environment variables HTTP{S}_PROXY

Tested with mitmproxy.

HTH.

[jandubois]

Why use only HTTP_PROXY but not http_proxy evnironment variables? I find that most apps either only support the lowercase variant (git, curl), or both, but only supporting uppercase seems rare.