Apm: add proxy.pac support

[dallyingllama]

Browsers are currently supporting automatic proxy configuration. Many companies use this to manage thier proxy implementations.

See https://en.wikipedia.org/wiki/Proxy_auto-config

The proxy auto-config (PAC) file tells the web browsers how to automatically choose the appropriate proxy server to use for fetching a given URL. The PAC file itself is JavaScript

"A PAC file contains a JavaScript function “FindProxyForURL(url, host)”. This function returns a string with one or more access method specifications. These specifications cause the user agent to use a particular proxy server or to connect directly."

This would be quite handy to be able to configure (as we can configure a proxy.pac) in our browsers so that we can easily add packages to atom from the internet.

Here are a couple of links related to people having problems related to this.

http://stackoverflow.com/questions/25997440/how-to-configure-atom-to-use-proxy-file-pac-settings
and somewhat related ...
https://discuss.atom.io/t/error-running-apm-install-behind-proxy/14812
https://discuss.atom.io/t/manually-install-package/9251

And some links regarding pac file implementations.
https://developer.mozilla.org/en-US/docs/Mozilla/Projects/Necko/Proxy_Auto-Configuration_%28PAC%29_file
https://static.googleusercontent.com/media/www.google.com/de//support/enterprise/static/postini/docs/help_center/PAC_Guide_V.1.pdf

Current and suggested behavior
The suggestion would be to add in approriate place something similar to adding a proxy manually in one of the config or ini files.

Steps which explain the enhancement

Perhaps in addition to these standard proxy configurations there could be one that links to the proxy.pac. In Firefox you can see these settings under proxy settings.
http_proxy=http://yourproxy.com:80
https_proxy=https://yourproxy.com:443
auto_proxy=http://proxy.local/proxy.pac

Why would the enhancement be useful to most users
The proxy.pac javascript can then be interpreted just as browsers interpret them. Then all users that work behind firewalls like this will just need to know where the auto proxy file is located and add this to the config.
I would love to not have to jump through so many hoops to get my atom application with the right packages installed. (i.e. on another computer then via usb stick etc. etc.). Perhaps there are other users that will appreciate this feature as well. Thank you for considering it.

[Workshop2]

Due to proxy, I also can't use APM. Please look into this :+1:

[burneyhoel]

I cannot update packages at work because of this. I would love to see the auto_proxy=http://proxy.local/proxy.pac implemented. I am currently working to develop packages for my team and convert them to atom but cannot until this change is completed. Thanks.

[RossWS]

Same boat here would love to see this.

[edevog]

Hello, I am also having this problem. Has it been resolved?
Thanks!

[HartmannTim]

I would also be happy to see an implementation of proxy autoconfig. It can be very useful because many companies are using the autoconfiguration.