Notify users when update is available about cabal-cli

[hackergrrl]

Brainstorming integration UX:

• showing a npm-style notification box once someone quits the app (I personally really dislike these)
• showing a message in the !status channel on cli init, or if the cli periodically checks for updates
• show a short notification message somewhere on the top bars

This approach will only work for npm installs, since installs from e.g. a package manager that didn't come from npm may not be in sync with our release cycles. It would be rad if we had an /upgrade command that could update in place, not unlike running youtube-dl -U.