Added ability to send light commands (on, off, set colors, etc.)

This implementation is barebones and only does what I was able to find easily in the official app's source code. I'd love to do more with this, so any pull requests adding functionality would be welcomed.

Closes https://github.com/parnic/node-screenlogic/issues/4

messages/SLLightControl.js

@@ -0,0 +1,25 @@
+'use strict';
+
+const SLMessage = require('./SLMessage.js').SLMessage;
+
+const MSG_ID = 12556;
+
+exports.SLLightControl = class SLLightControl extends SLMessage {
+  constructor(controllerIndex, command) {
+    super(0, MSG_ID);
+
+    this.controllerIndex = controllerIndex;
+    this.command = command;
+  }
+
+  encode() {
+    this.writeInt32LE(this.controllerIndex || 0);
+    this.writeInt32LE(this.command || 0);
+
+    super.encode();
+  }
+
+  static getResponseId() {
+    return MSG_ID + 1;
+  }
+};