As someone who works on an RSS crawler day to day, this is much more difficult than you think because of how poorly most people implement RSS.

If everyone stuck to the standards and did things the same way, it would be easy. If you can't account for these errors your reader wont work with 80% of feeds, and nobody will use it.

Rss feeds are usually programmed more poorly than a website itself, because it's one of those features that comes from a discussion that usually goes: "well everyone has an rss feed, so we should have one, so make one". Yet they know people will rarely use it.

You could make it work for the popular CMS's but even between those there are standards problems. The majority of the problems are in storing the stuff that's crawled.