Feature: Add PagerDuty as an option

[revilo951]

It'd be awesome to be able to use PagerDuty with shoutrrr

https://developer.pagerduty.com/docs/ZG9jOjExMDI5NTgw-events-api-v2-overview

[simskij]

Feel free to have a stab at it if you want! 👍🏼

[revilo951]

If I knew even the smallest amount of go, I would! Unfortunately, I've only ever done python stuff so far, and precious little of that.

[noelob]

Submitted #468 to add PagerDuty support