add backup_file_format option (default value)

grafana_backup/conf/grafanaSettings.json

@@ -3,14 +3,16 @@
     "debug": true,
     "verify_ssl": true,
     "backup_dir": "_OUTPUT_",
+    "backup_file_format": "%Y%m%d%H%M",
     "pretty_print": false
   },
   "grafana": {
     "url": "http://localhost:3000",
-    "token": "",
+    "token": ""
     "search_api_limit": 5000,
     "default_password": "00000000",
     "admin_account": "",
     "admin_password": ""
   }
 }
+