added option to redirect all anonymous users to login page

- new setting *force_login* added and documented
- if both, *force_login* and *hide_public_links* are set to true, all requests
  (except for the feeds) are redirected to the login page

doc/md/Shaarli-configuration.md

@@ -90,6 +90,7 @@ _These settings should not be edited_
 
 - **default_private_links**: Check the private checkbox by default for every new link.  
 - **hide_public_links**: All links are hidden while logged out.  
+- **force_login**: if **hide_public_links** and this are set to `true`, all anonymous users are redirected to the login page.
 - **hide_timestamps**: Timestamps are hidden.
 - **remember_user_default**: Default state of the login page's *remember me* checkbox
     - `true`: checked by default, `false`: unchecked by default
@@ -194,6 +195,7 @@ _These settings should not be edited_
     "privacy": {
         "default_private_links": true,
         "hide_public_links": false,
+        "force_login": false,
         "hide_timestamps": false,
         "remember_user_default": true
     },