add configuration option to add extra resources to cache manifest

In order to add extra resources to the appcache manifest, I added an extra configuration option 'extraResources'. This options accepts an array of urls, that are appended after the bundle manifest cacheable assets.

    Meteor.AppCache.config(
      {
        extraResources: ['url_1', 'url_2]
      }
    );

I needed this functionality to add maptiles of a tiling service (cloudmade.com) to my appcache, to be able to navigate through maps without connectivity.