Cannot create an aggregate event handler in JSON

I tried to define an `aggrgate` event handler in JSON but Kapacitor rejects it.

Here is the JSON:
```
{
"id": "foo-aggr",
"topic": "foo",
"kind": "aggregate",
"options": {
"interval": "10m",
"topic": "bar"
}
}
```

There error I get is:
```
failed to create handler: failed to decode options into *alert.AggregateHandlerConfig: 1 error(s) decoding:

* 'interval' expected type 'time.Duration', got unconvertible type 'string'
```

Kapacitor should convert the JSON string to a duration.

Start with the JSON handler decoding path and the alert.AggregateHandlerConfig type named in the error; reproduce the failure using the supplied aggregate handler JSON. Done means the interval value is accepted as a duration and the aggregate event handler can be created successfully, with a regression test if the existing decoder tests cover this path.

Written by the indexing model from the issue text.