Add a feature to hide articles when they are read

This is a new reading option to hide articles when they are read. The hidding process occurs when the article is left for an other article. This way, even when the article is marked as read on opening, it is hidden only while navigating to an other article.
I'm not really happy with the behavior when the "mark while scrolling" option is enabled. Please review.
It is missing the i18n since we're not supposed to push them before it exists on i18n.freshrss.org. Or maybe I misunderstood the process.

See #476