Support reading Atom feeds

The version 2.9.0 has introduced the capability to read RSS feeds and add new items to specified collection automatically, which is very handy. Unfortunately, the app fails to read some Atom feeds, namely those ones with features that are beyond RSS spec thus unsupported by 'rss-parser' package.

It would be really great to have an opportunity to subscribe to any Atom feed.

Probably, it might be achievable by replacing the 'rss-parser' package with 'feedparser', which seems to support RSS, Atom, and RDF feeds