[i18n_subsites] Add the possibility of hiding pages or articles separately


https://github.com/amic-github/pelican-plugins/commit/65334575cbfb43eb9fb96f7742e5af205611ee60

It may be useful to hide only untranslated articles, or only untranslated pages.
For instance one can consider the page lang consistency to be more important on a site, while allowing some articles to be in another language.

I used to hide pages manually with the theme, but maybe this way is more appropriate.