Add support for substr that matches PHP's slightly wider functionality

Author: jwadhams

bower.json

@@ -1,6 +1,6 @@
 {
   "name": "json-logic-js",
-  "version": "1.2.0",
+  "version": "1.2.1",
   "homepage": "https://github.com/jwadhams/json-logic-js",
   "authors": [
     "Jeremy Wadhams <jwadhams@dealerinspire.com>"

logic.js

@@ -81,6 +81,14 @@ http://ricostacruz.com/cheatsheets/umdjs.html
     "cat": function() {
       return Array.prototype.join.call(arguments, "");
     },
+    "substr":function(source, start, end) {
+      if(end < 0){
+        // JavaScript doesn't support negative end, this emulates PHP behavior
+        var temp = String(source).substr(start);
+        return temp.substr(0, temp.length + end);
+      }
+      return String(source).substr(start, end);
+    },
     "+": function() {
       return Array.prototype.reduce.call(arguments, function(a, b) {
         return parseFloat(a, 10) + parseFloat(b, 10);

package.json

@@ -1,6 +1,6 @@
 {
   "name": "json-logic-js",
-  "version": "1.2.0",
+  "version": "1.2.1",
   "description": "Build complex rules, serialize them as JSON, and execute them in JavaScript",
   "main": "logic.js",
   "directories": {