Added AMD friendly piece of code.

src/showdown.js

@@ -1442,3 +1442,12 @@ var escapeCharacters_callback = function(wholeMatch,m1) {
 
 // export
 if (typeof module !== 'undefined') module.exports = Showdown;
+
+// stolen from AMD branch of underscore
+// AMD define happens at the end for compatibility with AMD loaders
+// that don't enforce next-turn semantics on modules.
+if (typeof define === 'function' && define.amd) {
+    define('showdown', function() {
+        return Showdown;
+    });
+}