contrib: download files from RSS

Based on #831, ignore the first 4 commits.

### From the README

This tool downloads files pointed by RSS feeds.

It fetches the content of the RSS feed, the it filters it according to custom patterns and to existing folders on disk. Selected elements will be downloaded to the given folder.

This tool must be compiled with its configuration in order to work. See the `sample_config.nit` in `src/`.

#### Features and TODO

- [x] Download & parse RSS file
- [x] Generate regular expressions from local folders
- [x] Auto download files matching regular expressions
- [x] Memory to prevent double downloads
- [ ] Blacklist
- [ ] Use a cleaner RSS parser
- [ ] Tolerate more feed formats
au top-level:

~~~
fun feed_url: String do return "http://example.org/feed.rss"
~~~

ici: utilise `feed_url`.

A la compile:

~~~
$ nitc sample_config.nit -D feed_url=http://ailleurs.org/feed.rss
~~~