Add "use strict" to most core nodes.

(skipping ones that may have other work in progress)

nodes/core/core/80-template.js

@@ -15,6 +15,7 @@
  **/
 
 module.exports = function(RED) {
+    "use strict";
     var mustache = require("mustache");
     var util = require("util");
     var fs = require('fs');

nodes/core/core/90-comment.js

@@ -15,6 +15,7 @@
  **/
 
 module.exports = function(RED) {
+    "use strict";
     function CommentNode(n) {
         RED.nodes.createNode(this,n);
     }

nodes/core/core/98-unknown.js

@@ -15,6 +15,7 @@
  **/
 
 module.exports = function(RED) {
+    "use strict";
     function UnknownNode(n) {
         RED.nodes.createNode(this,n);
     }