Update P1Accessory.js

- Add accessory for Capacity tariff, see #69.

lib/P1Accessory.js

@@ -33,6 +33,7 @@ class P1Accessory extends homebridgeLib.AccessoryDelegate {
   }
 
   static get Electricity () { return Electricity }
+  static get ElectricityCapacity () { return ElectricityCapacity }
   static get Gas () { return Gas }
   static get Water () { return Water }
 }
@@ -60,6 +61,22 @@ class Electricity extends P1Accessory {
   }
 }
 
+class ElectricityCapacity extends P1Accessory {
+  constructor (platform, data, name = 'Electricity Capacity') {
+    data.id += 'C'
+    super(platform, name, data)
+    this.service = new P1Service.ElectricityCapacity(this, data)
+    this.historyService = new homebridgeLib.ServiceDelegate.History(
+      this, {
+        averagePowerDelegate: this.service.characteristicDelegate('power')
+      }
+    )
+    if (!this.platform.config.outlet) {
+      this.dummyService = new homebridgeLib.ServiceDelegate.Dummy(this)
+    }
+  }
+}
+
 class Gas extends P1Accessory {
   constructor (platform, data) {
     super(platform, 'Gas', data)