Add filter for process_product_file_download_paths()

Adding a new filter to the process_product_file_download_paths() method so that plugins can intercept the file list and decided whether permissions should be updated. This will be a huge help to sites that sometimes change downloadable file names but don't want existing permissions to be reset. A plugin can parse the list and remove files from the $downloadable_files array if a particular file should not have permissions reset.  This can help resolve issue https://github.com/woothemes/woocommerce/issues/4180 and issue https://github.com/woothemes/woocommerce/issues/4669 without changing the long-standing downloadable files processing behavior of WooCommerce.