Updated exports syntax to work like README demonstrates

src/showdown.js

@@ -1335,4 +1335,4 @@ var escapeCharacters_callback = function(wholeMatch,m1) {
 } // end of Showdown.converter
 
 // export
-if (typeof exports != 'undefined') exports = Showdown;
+if (typeof exports != 'undefined') exports.Showdown = Showdown;