Add an update nudge to the CLI version

[gaearon]

The Electron version (packages/react-devtools) is often installed globally, so we should add an updating nudge that prints a message when a newer version is available.

update-notifier is a good package that already knows how to do this so we should just integrate it there.

[tsriram]

I'll take a look at this.

[sumnerevans]

@gaearon, @tsriram, Is there a reason this is still open? If yes, is there anything that I can do to help complete this?

[gaearon]

Sorry, I think it was done and I forgot to close the issue. Thanks for reminding!