Added flattr to the feed

This adds flattr to the feed ... The necessary infos for the auto URL stuff need to be added to the `podcast.yml`

```yaml
---
flattr:
  user_id: klaustopher
  language: de_DE
  tags:
    - podcast
    - nerdkunde
    - technology
    # See https://api.flattr.com/rest/v2/categories.txt
  category: audio
```

## Still to do
- [ ] Add tests for the Auto-Submit-URL generation to `Podcast` and `Episode`
- [ ] Fix `Podcast#deep_link` since it only handles GST-Kitchen URLs (@tisba, please let's talk about this)