Derive feed from HTML (#10)

- Prevent duplicate feed subscriptions
- Move FeedItem to its own file

src/app-config.php

@@ -1,7 +1,7 @@
 <?php
 
 /** The current Feed Reader Central version */
-const FRC_VERSION = '1.0.0-alpha4';
+const FRC_VERSION = '1.0.0-alpha5';
 
 spl_autoload_register(function ($class) {
     $file = implode(DIRECTORY_SEPARATOR, [__DIR__, 'lib', "$class.php"]);