Allow each partials dir to be configured

Since each partials dir can be [specified as an object](https://github.com/ericf/express-handlebars/pull/70), each dir can be configured differently. This is useful for example to always cache distributed partials, that don't need to be reloaded at each request in development:

```javascript
var hbs_config = {
  extname: '.hbs',
  partialsDir: [
    '/.../views',
    {dir: '/.../node_modules/third-party/views', namespace: 'third-party', cache: true}
  ],
  handlebars: Handlebars
};
var handlebars = express_handlebars.create( hbs_config );
```