Add missing compdef call

The vanilla version assumes that the completion script is generated once
and then put in the zsh completion script folder. This has advantages,
such as better caching, but it breaks the most common use-case
`source <(rakkess completion zsh)`

This commit adds a compdef call which re-enables this usage.

Also see spf13/cobra#887

cmd/completion.go

@@ -18,6 +18,8 @@ package cmd
 
 import (
 	"fmt"
+	"io"
+
 	"github.com/sirupsen/logrus"
 	"github.com/spf13/cobra"
 )
@@ -38,6 +40,8 @@ const (
 
 	Additionally, you may want to output the completion to a file and source in your .bashrc
 `
+
+	zshCompdef = "\ncompdef _rakkess rakkess\n"
 )
 
 var completionCmd = &cobra.Command{
@@ -58,7 +62,7 @@ var completionCmd = &cobra.Command{
 		case "bash":
 			err = rootCmd.GenBashCompletion(out)
 		case "zsh":
-			err = rootCmd.GenZshCompletion(out)
+			err = getZshCompletion(out)
 		}
 		if err != nil {
 			logrus.Fatal(err)
@@ -69,3 +73,11 @@ var completionCmd = &cobra.Command{
 func init() {
 	rootCmd.AddCommand(completionCmd)
 }
+
+func getZshCompletion(out io.Writer) error {
+	if err := rootCmd.GenZshCompletion(out); err != nil {
+		return err
+	}
+	_, err := io.WriteString(out, zshCompdef)
+	return err
+}