Add more possible matches for automatic tagging

[Thaodan]

This PR adds more possibly matches for make-tagger. The first change is to add existing attributes from elfeed-feed i.e. feed-author. Matching against the author is very useful for example for Youtube feeds where the url isn't enough of a useful indicator.

The other two new matches allow matching against the enclosure or content-type.

The last change makes it possible to match against entries from the meta of feed or entry.

I think the author data should also go to the feed entry since it isn't possible to match against author for RSS feeds without it unless matching directly against meta.