more flexible 'index' option for directory handler

This is a simple backwards-compatible extension to the directory handler, allowing the `index` option to be either a `boolean` (as before) or a `string` (new option).

This allows the index file to be configured to be something other than `'index.html'`, without breaking any previous behaviour.

There's also Joi validation to ensure nothing unsafe can be used.

EDIT: Added units tests.