adding a GenZshCompletionFile function to mirror bash_completion (#529)

zsh_completions.go

@@ -4,9 +4,21 @@ import (
 	"bytes"
 	"fmt"
 	"io"
+	"os"
 	"strings"
 )
 
+// GenZshCompletionFile generates zsh completion file.
+func (cmd *Command) GenZshCompletionFile(filename string) error {
+	outFile, err := os.Create(filename)
+	if err != nil {
+		return err
+	}
+	defer outFile.Close()
+
+	return cmd.GenZshCompletion(outFile)
+}
+
 // GenZshCompletion generates a zsh completion file and writes to the passed writer.
 func (cmd *Command) GenZshCompletion(w io.Writer) error {
 	buf := new(bytes.Buffer)