Healthcheck hook

[cmj2002]

Certspotter now has 3 event types to trigger a script: discovered_cert, malformed_cert, error. If it could also trigger during the health check, that would be even better, as it would allow us to determine whether certspotter is still running by having it ping an external monitoring service. In some extreme cases (such as OOM killer, virtual machine kernel panic, or even power outage), certspotter may not have the opportunity to send error reports.