Pushover Notifications Driver [Slightly Enhanced]

[stephack]

@ogiewon
Sent Pull request for Emergency Notification based on request below.

Request: Pushover Emergency Priority

I’m using Pushover since the last update (thanks for the tip!), and I like it! The ability to specify L, M or H for each message is great, and a nice way to differentiate between different notifications. I noticed that in the API description (Pushover: API) additional levels are described, among them “Emergency”. To me it sounds perfect for alarms, leaks and that kind of more serious “threats”, as it continues to notify until user action is taken. So, TL;DR. Would it be possible to add [E] for…