Apply color by annotation

The default Alertmanager<->OpsGenie integration is using 'annotation.priority' as it's preferred field to determine severity-level.
I would like to show these alerts in Karma and match the priority-color according to OpsGenie colors.
However, the configuration of Karma seems to only allow 'labels' as colorable.

I have 2 basic ideas:

1. Expand the config to allow of colorizing 'annotations'
2. Add a setting/function to 'promote' certain annotations into labels (thus using the existing config)
   (this could also allow a 'demote'-setting to convert a label into an annotation)

I'm willing to try to do a PR for option 2, but only if you feel it's a good-enough option