* Add a new Pushover provider (#129)
- Adds new provider named Pushover with corresponding tests
- Adds Pushover as a provider to the configuration and adjusts test accordingly
- Adds Pushover to alerting_test.go, provider.go and type.go
- Updates the readme with configuration details
* Correct import order
* Fix some missing pushover references
* Apply suggestions from code review
* Rename application-key to application-token for Pushover
---------
Co-authored-by: TwiN <twin@linux.com>
