Merge: contrib/rss_downloader: compressed feeds and report errors

This PR adds a few features to rss_downloader to support more RSS feeds. It can be configured to decompress the feed and to customize the tag to find the download link. The output has be improved, both on normal behavior and on errors.

As a bonus, add the program I used to debug the RSS feeds as an example for the `dom` module.

Pull-Request: #1590
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>