Integrate with Github Issues

[pc-alves]

Create a notification plugin for Github issues.
This will allow using Github as a repository for low severity alerts that can be analyzed and prioritized during business hours.
An alternative to email notifications that can be easily managed by teams

[mohabusama]

Feature to be implemented here: zalando-zmon/zmon-worker#381

[mohabusama]

Closing as it needs to be implemented in the worker.