Auto merge of #3598 - pornel:proxyconf, r=alexcrichton
Auto merge of #3598 - pornel:proxyconf, r=alexcrichton

Example proxy config

#3596

nipunn1313/cargo,tsurai/cargo,rustyhorde/cargo,durka/cargo,eminence/cargo,nipunn1313/cargo,achanda/cargo,tshepang/cargo,tsurai/cargo,tshepang/cargo,tsurai/cargo,eminence/cargo,rust-lang/cargo,achanda/cargo,achanda/cargo,rustyhorde/cargo,nipunn1313/cargo,eminence/cargo,vojtechkral/cargo,durka/cargo,rust-lang/cargo,tsurai/cargo,eminence/cargo,sbeckeriv/cargo,nipunn1313/cargo,durka/cargo,sbeckeriv/cargo,tsurai/cargo,durka/cargo,rust-lang/cargo,vojtechkral/cargo,sbeckeriv/cargo,rustyhorde/cargo,rust-lang/cargo,vojtechkral/cargo,tshepang/cargo,vojtechkral/cargo,sbeckeriv/cargo,tshepang/cargo,rustyhorde/cargo,achanda/cargo