Add --label-key-state to override the exit state

[martialblog]

If this flag is set the plugin looks for warning/critical/ok in the label 'icingaState'

  -S, --label-key-state string      
Use the given AlertRule label to override the exit state for firing alerts.
If this flag is set the plugin looks for warning/critical/ok in the provided label key

See #112

[martialblog]

I'm not yet sure if a single flag is enough, we could also for example just say if the -K, --state-labels-key is set, then it is used to override the exit state