Added ability to specify FALLBACK

The FALLBACK section of the manifest file allows specifying a fallback page when a resource is unavailable:

> An optional section specifying fallback pages if a resource is inaccessible. The first URI is the resource, the second is the fallback used if the network request fails or errors. Both URIs must from the same origin as the manifest file. You can capture specific URLs but also URL prefixes. "images/large/" will capture failures from URLs such as "images/large/whatever/img.jpg".

This pull request allows users to pass an array of lines to be added to the FALLBACK section.