Specify a file to configure the remote downloader with. This file consists of lines, each of which starts with a directive (`allow`, `block` or `rewrite`) followed by either a host name (for `allow` and `block`) or two patterns, one to match against, and one to use as a substitute URL, with back-references starting from `$1`. It is possible for multiple `rewrite` directives for the same URL to be give, and in this case multiple URLs will be returned.