Add let keyword in ES6

https://egghead.io/lessons/the-let-keyword#/tab-code

es6/let_keyword.js

@@ -0,0 +1,66 @@
+// Original
+var message = "hi";
+
+{
+  var message = "bye";
+}
+
+console.log(message); //outputs "bye"
+
+// Let
+let message = "hi";
+
+{
+  let message = "bye";
+}
+
+console.log(message); //outputs "hi"
+
+// Example
+var fs = [];
+
+for(var i=0; i <10; i++) {
+  fs.push(function() {
+    console.log(i);
+  })
+}
+
+fs.forEach(function (f) {
+  f();
+})
+
+// Output
+// 10
+// 10
+// 10
+// 10
+// 10
+// 10
+// 10
+// 10
+// 10
+// 10
+
+var fs = [];
+
+for(let i=0; i <10; i++) {
+  fs.push(function() {
+    console.log(i);
+  })
+}
+
+fs.forEach(function (f) {
+  f();
+})
+
+// Output
+// 0
+// 1
+// 2
+// 3
+// 4
+// 5
+// 6
+// 7
+// 8
+// 9