Adding PagerDuty Notifier, closes #567

This notifier opens an incident in PagerDuty when a backup model fails
or completes with a warning. A successful run will resolve an open
incident for that model.

Optionally, the notifier can be configured to resolve on a warning,
rather than triggering an incident. This might be wise when multiple
notifiers are in use (so warnings don't go unnoticed) and users want a
the next successful run to resolve the previously triggered incident.
Escape username and password for postgres database params, #581
Add pagerduty dependency for 3f852e862b7e0dffccab35f14c4fc52774c299f7
Release v4.0.5
Add SQLite3 support, closes #580
Release v4.0.6 [ci skip]
Mention the backup features repository and its purpose [ci skip]
Add DataDog notifier