Add completion for cargo, the rust build tool

Copy of the official repository:
https://github.com/rust-lang/cargo/tree/master/src/etc

Signed-off-by: Marc Cornellà <marc.cornella@live.com>

plugins/cargo/cargo.plugin.zsh

-function _cargo_commands() {
-    local ret=1 state
-    _arguments ':subcommand:->subcommand' && ret=0
-
-    case $state in
-      subcommand)
-        subcommands=(
-          "build:Build the current project"
-          "clean:Clean up after a build"
-          "help:Help about available commands"
-          "new:Create a new project"
-          "test:Run the tests"
-          "update:Updates list of known packages"
-          "run:Builds and runs the currecnt project"
-        )
-        _describe -t subcommands 'cargo subcommands' subcommands && ret=0
-    esac
-
-    return ret
-}
-
-compdef _cargo_commands cargo