[Feature Request] Add ability to specify alert handlers per alert.

Please allow users to add handlers per alert on top of global configuration:

This could use exactly the same parameters but within alert definition:

```
alerts: [
  {
    …
    "critical_handlers": ["log", "smtp"],
    "warning_handlers": ["log", "smtp"],
    "normal_handlers": ["log", "smtp"],
    …
  }
]
```

Thanks!
