OpsGenie2 - Allow arbitrary Alert Tags

The OpsGenie V2 API `/alerts` endpoint supports supplying a list of arbitrary tags in the `tags` JSON body field when submitting a POST request to create an alert. It would be nice if kapacitor exposed this functionality in the OpsGenie2 event handler.

```
var ogTags = ['Awesome', '{{ index .Tags "awesome_tag" }}']
stream
|alert()
.opsGenie2()
.tags(ogTags)
.teams('AwesomeTeam')
```

https://docs.opsgenie.com/docs/alert-api#section-create-alert

Locate the OpsGenie2 event handler and its alert-creation request path, then compare the existing alert configuration methods with the OpsGenie V2 POST /alerts API. Done means the handler accepts a tags list such as `.tags(ogTags)` and forwards arbitrary values in the request body.

Written by the indexing model from the issue text.