add webhooks or plugin API to notify on record/zone changes

[luto]

Push events like new zones or changed records to external systems. Either via a simple plugin API, which calls either python functions or executables or better via webhooks (or both). Think about filter levels (call for all zones, call for all zones in tenant or even for single domains?). Make at least the webhooks configurable in web interface.

This could be used to build external auditing or fun additions like a slack integration.

Maybe tie into #7.