Add other notification methods

[TimBanks]

Having push notification methods for pagerduty, email, prometheus, etc would add to the versatility of this feature.

[dbmurphy]

@TimBanks You might want to look at the MDB_1.0 branch as we are doing more of the modular work to make this easier for people to contribute new methods. The modular notifications should merged today, but we need to do some config file magic before it will be buildable

[timvaillancourt]

Created #243 and #244 for PagerDuty and Email notifications.

Please open specific issues for any other requests for methods, this request is too large/vague.