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
Je lis le comment, tout va bien.

Je lis la signature et je me demande pourquoi diantre ça prend un HashSet et pas un simple Array. Donc soit le commentaire n'en dit pas assez sur pourquoi les lignes du log doivent être uniques et comment récupérer l'ordre. Soit y'a un truc que je comprend pas.