a bug about options！

call:
var converter = new Showdown.converter({ extensions: 'github' });

This will cause an exception：
uncaught exception: Extension 'undefined' could not be loaded. It was either not found or is not a valid extension.

and I found that this problem can be avoided use:
var converter = new Showdown.converter({ extensions: ['github'] });

So I submitted this update to resolve the issue