Add feed preview cmd

[lmas]

Like the discover or regexp command, but instead shows a preview list of all items in a RSS/Atom feed.

Example use case: discover what kind of content would be discovered by the tool, for the feed https://whimsical.club/feed.xml.

For this feed (at this time) the link and title tags should be grabbed automatically (by default, which is technically correct) but if we also print the other tags, we would notice that the content tag looks more interesting instead of title (same url as in link). Then we could add a custom regexp rule to instead grab link and content and use them in the feedloggr feed.

Hmm content is already fetched automatically too, so might want to add a feed setting that let's one add priority to content and let the tool swap it with title or something.