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"
      }
   }