extensions should be a list, not a string.

This confused me a for a bit until I realised what was wrong.

README.md

@@ -110,7 +110,7 @@ Showdown allows additional functionality to be loaded via extensions.
 <script src="src/showdown.js" />
 <script src="src/extensions/twitter.js" />
 
-var converter = new Showdown.converter({ extensions: 'twitter' });
+var converter = new Showdown.converter({ extensions: ['twitter'] });
 ```
 
 ### Server-side Extension Usage