Add a new configuration option

Before, when you selected the option to display only unread articles, it shows read articles if no unread article were found.
Now, this option is renamed to include information on fallback behavior and a new option is created to have the "only unread" behavior

See #551

It is missing the german translation
Merge branch 'reading-conf' of github.com:aledeg/FreshRSS into aledeg-reading-conf
Coding style + i18n

Change "Show only unread or read if no unread" into "Adjust showing". It
is less explicit but shorter.
Merge branch 'aledeg-reading-conf' into dev