Add zsh completion command.

Fixes #862

Author: tpounds

pkg/commands/completion.go

@@ -1,6 +1,7 @@
 package commands
 
 import (
+	"fmt"
 	"os"
 
 	"github.com/pkg/errors"
@@ -17,16 +18,36 @@ func (e *Executor) initCompletion() {
 	bashCmd := &cobra.Command{
 		Use:   "bash",
 		Short: "Output bash completion script",
-		RunE:  e.executeCompletion,
+		RunE:  e.executeBashCompletion,
 	}
 	completionCmd.AddCommand(bashCmd)
+
+	zshCmd := &cobra.Command{
+		Use:   "zsh",
+		Short: "Output zsh completion script",
+		RunE:  e.executeZshCompletion,
+	}
+	completionCmd.AddCommand(zshCmd)
 }
 
-func (e *Executor) executeCompletion(cmd *cobra.Command, args []string) error {
+func (e *Executor) executeBashCompletion(cmd *cobra.Command, args []string) error {
 	err := cmd.Root().GenBashCompletion(os.Stdout)
 	if err != nil {
 		return errors.Wrap(err, "unable to generate bash completions: %v")
 	}
 
 	return nil
 }
+
+func (e *Executor) executeZshCompletion(cmd *cobra.Command, args []string) error {
+	err := cmd.Root().GenZshCompletion(os.Stdout)
+	if err != nil {
+		return errors.Wrap(err, "unable to generate zsh completions: %v")
+	}
+	// Add extra compdef directive to support sourcing command directly.
+	// https://github.com/spf13/cobra/issues/881
+	// https://github.com/spf13/cobra/pull/887
+	fmt.Println("compdef _golangci-lint golangci-lint")
+
+	return nil
+}