Apply Chromium proxies to env vars for apm

[damieng]

Description of the Change

Chromium does a good job at reading system proxy settings which means a lot of what Atom needs to do goes through the correct proxies.

The apm command line being a regular node app and not an Electron/Chromium app does not read or have access to these proxy settings and so fails when trying to connect if you are using a proxy.

This change determines what http and https proxy servers to use based on Chromium's session.resolveProxy call with atom.io and then sets the http_proxy and https_proxy environment variables that apm then uses.

Alternate Designs

Alternative designs include;

1. Moving apm functionality entirely into Atom - too much work at this time and would mess up command-line scripting
2. Editing the apm.rc file with the proxy settings - too destructive

Benefits

People behind proxy servers can use apm with no effort.

Possible Drawbacks

Users with manually set http_proxy and https_proxy environment variables will find we override their preferences. They can switch off the auto-detect proxy in settings.

[winstliu]

callback() unless atom.config.get('core.useProxySettingsWhenCallingApm')

seems cleaner.

[winstliu]

Also, this looks like it only gets used with runCommand, so maybe setProxyServers can be moved to inside runCommand?

[damieng]

I originally had it in runCommand but the problem is that runCommand immediately passed back a BufferedProcess object that things attach to - this being async it couldn't do that immediately. Breaks everywhere.