teach ntfy webpush to write the keys to a file

cmd/webpush_test.go

@@ -14,6 +14,13 @@ func TestCLI_WebPush_GenerateKeys(t *testing.T) {
 	require.Contains(t, stderr.String(), "Web Push keys generated.")
 }
 
+func TestCLI_WebPush_WriteKeysToFile(t *testing.T) {
+	app, _, _, stderr := newTestApp()
+	require.Nil(t, runWebPushCommand(app, server.NewConfig(), "keys", "--key-file=key-file.yaml"))
+	require.Contains(t, stderr.String(), "Web Push keys written to key-file.yaml")
+	require.FileExists(t, "key-file.yaml")
+}
+
 func runWebPushCommand(app *cli.App, conf *server.Config, args ...string) error {
 	webPushArgs := []string{
 		"ntfy",