Archive articles after download (and other minor refinements)

* Add an option to archive downloaded articles
* Make metadata before each article an option
* Remove feed (magazine section) if it doesn't have any article
* Abort if no article found (to eliminate empty books)

I also set the browser to support referrer but I don't know if it actually has any effect. There are strange redirections when downloading an article, but the effective way to solve the "recursion limit reached" issue if to use single thread and to increase the delay between downloads.