Add entry permanently to local cache.

        Parameters
        ----------
        resource_url : string
            Resource Url
        cache_id : string
            Unique cache identifier for resource

def cache_add(self, resource_url, cache_id):
        """Add entry permanently to local cache.

        Parameters
        ----------
        resource_url : string
            Resource Url
        cache_id : string
            Unique cache identifier for resource
        """
        # Add entry to cache index
        self.cache[resource_url] = cache_id
        # Write cache index content to database file
        with open(self.db_file, 'w') as f:
            for resource in self.cache:
                f.write(resource + '\t' + self.cache[resource] + '\n')