Support different Plugin Outputs per Service State

[frankfil]

The PR adds an option to support selecting the Plugin Output from different Annotation names based on the Service State.

Paraphrasing from the README:

Adds a new option called --alertmanager_pluginoutput_by_states & env var SIGNALILO_ALERTMANAGER_PLUGINOUTPUT_BY_STATES.

If set Signalilo will take the Service State name (ok, warning, critical, or unknown) and suffix this to the Annotation name when looking up the Annotation to use for the Plugin Output (for example: message_ok).

This allows you to configure multiple Annotations with different values that are then used with the corresponding Service State to set the Plugin Output.

If an Annotation is not found for that specific Service State then Signalilo will fall back to just using the Annotation name as configured. So this option is both backwards compatible and safe to enable without changing any alert configurations.

This is designed for alerts you send to non-technical people where having the same Plugin Output for the warning/critical and OK states can be confusing.

Checklist

• [x] Keep pull requests small so they can be easily reviewed.
• [x] Update the README.
• [x] Categorize the PR by setting a good title and adding one of the labels: bug, enhancement, documentation, change, breaking, dependency as they show up in the changelog

[frankfil]

Well that was confusing I thought it was from the latest master. I believe it's now OK, assuming I've done this correctly of course.

I'm still wrapping my head around some GIT stuff - too much of my life spent with SVN.