Honor npm config proxy settings

[bvandre]

Pull request to solve issue #65

[bpasero]

@bvandre thanks, good start. Would it make sense to also support npm_config_https_proxy if possible?

Also, there is another place where a request is being made: https://github.com/Microsoft/vscode-extension-vscode/blob/master/bin/test#L88

[bvandre]

I updated to add a check to see if the protocol was https and to use the correct proxy value. I am not sure if we want to add a fallback to use npm_config_http if npm_config_https_proxy is not found or not.

[bpasero]

Thanks