Ability to define custom handlers

The topics adds a nice interface to decouple alerting logic from tasks but it would be cool to have ability to have custom handlers in topics. For example one use case is Opsgenie. We have a opsgenie handler in kapacitor but it has an issue i-e when an alert goes from warning to critical opsgenie doesn't show that because both ciritical and warning alerts has the same 'alias' which are deduplicated by opsgenie deduplication engine.

If we have ability to define custom handlers we can then define our own logic in them. Currently we are forced to maintain our own fork of kapacitor which isn't nice for obvious reasons.

Start by tracing Kapacitor's existing topics interface and alert handlers, then compare how the Opsgenie handler handles warning-to-critical transitions. Define what a custom-handler extension must expose and verify that an implementation can support handler-specific deduplication behavior without requiring a maintained fork.

Written by the indexing model from the issue text.