Global option to mark updated articles as unread

https://github.com/FreshRSS/FreshRSS/issues/798

app/Controllers/feedController.php

@@ -342,7 +342,8 @@ class FreshRSS_feed_Controller extends Minz_ActionController {
 						} else {	//This entry already exists but has been updated
 							Minz_Log::debug('Entry with GUID `' . $entry->guid() . '` updated in feed ' . $feed->id() .
 								', old hash ' . $existingHash . ', new hash ' . $entry->hash());
-							$entry->_isRead(null);	//Change is_read according to policy. //TODO: Implement option
+							//TODO: Make an updated/is_read policy by feed, in addition to the global one.
+							$entry->_isRead(FreshRSS_Context::$system_conf->mark_updated_article_unread ? false : null);	//Change is_read according to policy.
 							if (!$entryDAO->hasTransaction()) {
 								$entryDAO->beginTransaction();
 							}

data/config.default.php

@@ -55,6 +55,10 @@ return array(
 	#	SimplePie, which is retrieving RSS feeds via HTTP requests.
 	'simplepie_syslog_enabled' => true,
 
+	# In the case an article has changed (e.g. updated content):
+	#	Set to `true` to mark it unread, or `false` to leave it as-is.
+	'mark_updated_article_unread' => false,
+
 	'limits' => array(
 
 		# Duration in seconds of the SimplePie cache,