Add runit exporter

It exports one metric with a label for the service name and labels for the services status:

      {
         "metric" : {
            "value" : [
               {
                  "value" : 1091,
                  "labels" : {
                     "want" : "down",
                     "name" : "test",
                     "normally_up" : "yes",
                     "state" : "down"
                  }
               },
               {
                  "value" : 8,
                  "labels" : {
                     "want" : "up",
                     "name" : "test",
                     "normally_up" : "yes",
                     "state" : "up"
                  }
               }
            ],
            "type" : "gauge"
         },
         "docstring" : "node_exporter: status of runit service.",
         "baseLabels" : {
            "name" : "node_service_status"
         }
      }

But maybe we should have one metric per service? Not sure about that.