update notifier introduced

[Update notifier](https://github.com/yeoman/update-notifier/) checks for latest version of defs every 1 day (default check interval for update notifier package) if installed defs version is the latest available.
notification, as well as checking itself may be suppressed by passing `--no-update-notifier` option.

I intentionally did not update the README - there's no section for options yet and I don't want to introduce one myself. Please start one at your own will to make options documentation easier.