Auto merge of #4543 - nerdrew:master, r=matklad
Auto merge of #4543 - nerdrew:master, r=matklad

teach zsh completion +{stable,beta,nightly}

e.g.: `cargo +nightly test`

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