Add new release checker

[jasonlong]

Implement a way for the app to check for new releases and show a notice for the user.

[jasonlong]

This looks like a pretty straightforward solution: https://github.com/megahertz/electron-simple-updater

[jasonlong]

I don't think this is going to happen after all since the autoupdater libraries I've looked at require the code to be signed. AFAIK this requires a paid Apple Developer account and Windows signing certs cost hundreds of dollar a year.

I'll revisit this if there's a free alternative.