allow the use of atom rss feeds

[cogfor]

Currently, News-please does not parse atom feeds. This PR fixes that.

[fhamborg]

That's nice. However, I currently don't think I see all the implications of this change, e.g., if later parts in the internal workflow would also work, since this PR only changes the filter criterion. There are also a few other things to consider, e.g., quite obvious that the variable names do not match anymore, e.g., the variable "re_rss" should be named "re_stream" (or something like that), since rss is not a subset of atom or vice versa. Did you test the full workflow?

Edit: Looks like there are some technical differences in the formats (rss vs atom) that require changes in later parsing of the stream, see https://www.quora.com/What-is-the-difference-between-rss-feed-and-atom-feed

[JeromeGill]

@fhamborg would you prefer a PR with atom support in a separate crawler (leaving the user to have to understand what type of feed they are consuming) or would you prefer "RssCrawler" refactored to handle both?

[fhamborg]

Hi Jerome! Thanks for helping with news-please :-) I guess it depends on the amount of similarities between both "procedures", e.g., if it was only a few lines being different, it would be fine to have a single class handling both RSS and Atom. The class should be called FeedCrawler maybe instead of RssCrawler. But if RSS and Atom actually required significantly different processing, then we should have two different classes.