Pushover notifier

This is a notifier for the [Pushover](https://pushover.net) notification service.
It automatically truncates the title (application_name) and message text to adhere to Pushover's limits.
Additionally, it allows setting the notification priority value (except priority 2) and custom notification sound as per the [Pushover API](https://pushover.net/api).
Let me know if you have any suggested changes, I will be happy to make them.