Print a message if a newer version is available #154

Hi there!

I found this feature very useful in the GitHub CLI application and npm.

I think it'll be useful for users who install drill using cargo.
And this will let them know that a new version is available and it's time to update.

Ready-made crates to do that:

• https://github.com/mgrachev/update-informer
• https://github.com/tarikeshaq/update-notifier