Fix#225/add proxy to publish functions

[Benny1007]

No description provided.

[bmanikm]

Thanks @Benny1007! Please let us know whether the Publish-Module cmdlet is working fine with the proxy settings.
I think, we need to execute the below commands to ensure that publish operation works fine with the proxy settings, right?

nuget.exe config -set http_proxy=http://my.proxy.address:port
nuget.exe config -set http_proxy.user=mydomain\myUserName
nuget.exe config -set http_proxy.password=mySuperSecretPassword

[bmanikm]

@Benny1007 Closing this PR for now. Please feel free to reactivate this PR when above https://github.com/PowerShell/PowerShellGet/pull/255#issuecomment-379033102 gets resolved.