Add pushover notification option for offline alerts

Right now only email and sms are allowable options for offline alert. Request is for notification parity as other alerts by adding pushover notification option to the offline alert.

Offline is important to find out about, currently I have to set a zapier alert on my email to catch teslafi's offline email and send a pushover. Would prefer not using zapier for this especially given teslafi already has pushover capability, just not for offline alert - yet! :)