Add update notifier

[phated]

Is this something people like and would want? We would use https://github.com/yeoman/update-notifier like yeoman does.

cc @sindresorhus

[sindresorhus]

We've found it to help considerably for keeping people up to date in Yeoman. Users would almost never upgrade the CLI unless they hit a problem, without the update notification.

[Romanx]

I'm in the 👍 group here too, showing people they can upgrade is useful.