Feature Request: Configurable alert priorities for Pushover alerts

We'd like to be able to set the Pushover alert priority independent of the Kapacitor alert level. Currently a critical alert will always be mapped to the "1" level in Pushover, as can be seen in the "priority" function here:

https://github.com/influxdata/kapacitor/blob/5d228b205dc4d5624b5b90208f320a1bbdc6e9c1/services/pushover/service.go#L136

We'd like to be able set this to a level of our choosing. For example:

.pushover()
.URL('http://example.com/grafana/{{ index .Tags "host" }}')
.URLTitle('Dashboard')
.Priority(-2)

Please let me know if any more info is required. Thanks.

Start in services/pushover/service.go, especially the priority function around line 136, and trace how the Pushover service is configured from the .pushover() entry point. Confirm how a configurable Priority(-2) value should override the Kapacitor alert level, then verify the resulting Pushover priority behavior with the relevant service tests if available.

Written by the indexing model from the issue text.