Use `update-notifier` to notify users when a new version is available

[FrancescoCioria]

requirements

We already used update-notifier successfully in hophop.

Here we should:

• notify user once if there is a new version
• notify user every time there is a new version

so, if user has 4.0.0 and we release 5.0.0

• user is notified

if we then release 5.1.0 and user hasn't yet updated

• user is notified again

specs

• update-notifier

misc

{optional: other useful info}