Add polyfills for String.trimLeft/Right for browsers without support.

demo/browser-polyfills.js

@@ -0,0 +1,26 @@
+"use strict";
+
+// Polyfills for browsers that do not support String.trimLeft/Right
+function trimLeftPolyfill() {
+  return this.replace(/^\s*/, "");
+}
+/* istanbul ignore if */
+if (!String.prototype.trimLeft) {
+  String.prototype.trimLeft = trimLeftPolyfill;
+}
+function trimRightPolyfill() {
+  return this.replace(/\s*$/, "");
+}
+/* istanbul ignore if */
+if (!String.prototype.trimRight) {
+  String.prototype.trimRight = trimRightPolyfill;
+}
+
+// Export for testing
+/* istanbul ignore else */
+if ((typeof module !== "undefined") && module.exports) {
+  module.exports = {
+    "trimLeftPolyfill": trimLeftPolyfill,
+    "trimRightPolyfill": trimRightPolyfill
+  };
+}

demo/default.htm

@@ -27,6 +27,7 @@
       <footer>Copyright &copy; 2015 by <a href="//dlaa.me/">David Anson</a></footer>
     </div>
   </div>
+  <script src="browser-polyfills.js"></script>
   <script src="../node_modules/markdown-it/dist/markdown-it.min.js"></script>
   <script src="require-stub.js"></script>
   <script src="markdownlint-browser.js"></script>