Track connectivity / dependancy preconditions for notifications.

[jtagcat]

Monitor connectivity to the general network. Allow adding multiple connectivity (depends on) targets (such as 172.20.0.1, iana.org, telegram.com).
If they are down, assume depending targets as unknown, don't send notifications for depending. Send notifications for the connectivity targets, if configured.

Said simply, dependancy precondition for sending a notification.

Before firing notifications for service (also 'back online'), check if connectivity works.
This avoids sending off a bunch of down (or only up, if the general network wasn't available) notifications, where the base connectivity is actually the problem.

[deefdragon]

Similar to #216

[jtagcat]

If the implementation includes 'everything depends on' / defaults, closing as featureful dupe.