Pushover title template variables

Hi,

It seems that the Pushover alert service does not support template variables in the `title` parameter.

Example use-case:
```
state_data
|window()
.period(150m)
.every(5m)
.fillPeriod()
|mode('code')
.as('state_code')
|alert()
.info(lambda: "state_code" <= 3)
.stateChangesOnly()

.message('')

.pushover()
.title('{{ .Level }} System Alert: {{ index .Tags "name" }} on {{ index .Tags "cloud" }}')
```

Would it be possible to support templating in the `title` parameter?

Start by locating the Pushover alert-service implementation and the handling of its title parameter. Compare that behavior with the template-variable example in the issue. Done means template variables render in Pushover titles when an alert is sent, with coverage for the example case.

Written by the indexing model from the issue text.