Option for app to quit on close window/s

[NicholasSloan]

Enable preference (or hidden preference via Term) for app to quit on closure of last window.

[karbassi]

Thanks @NicholasSloan for the suggestion. We're trying to keep the app very simple. If others want this feature, we'll add it to the list of features.

Please vote by putting a thumbs up on the issue.

[karbassi]

For anyone that wants to take on the PR, the code to enable this feature is:

func applicationShouldTerminateAfterLastWindowClosed(_ sender: NSApplication) -> Bool {
    return true
}

inside the AppDelegate.swift