Add configuration file override

Author: josbeir

README.md

@@ -129,7 +129,9 @@ The hub will be available at `http://localhost:3000/.well-known/mercure`.
 
 The plugin includes a default configuration file with all available options. The configuration is automatically loaded from the plugin's `config/mercure.php` file.
 
-Set the required environment variables in your `.env` file:
+You can also create a `config/app_mercure.php` file in your project which will be loaded after the plugin config file, allowing you to override any settings.
+
+Alternatively you can set a few environment variables in your `.env` file for quick setup. Refer to the `mercure.php` for a full list of predefined env variables:
 
 ```env
 MERCURE_URL=http://localhost:3000/.well-known/mercure
@@ -397,7 +399,7 @@ Publisher::publish($update);
 > **View Class Configuration:** By default, `ViewUpdate` uses CakePHP's automatic view class selection (your `AppView` if it exists, otherwise the base `View` class). You can override this by setting `view_class` in your configuration:
 >
 > ```php
-> // In config/mercure.php
+> // In config/app_mercure.php
 > return [
 >     'Mercure' => [
 >         'view_class' => \App\View\CustomView::class,

src/MercurePlugin.php

@@ -26,6 +26,11 @@ public function bootstrap(PluginApplicationInterface $app): void
 
         // Load plugin configuration
         Configure::load('Mercure.mercure');
+
+        // Load app specific config file.
+        if (file_exists(ROOT . DS . 'config' . DS . 'app_mercure.php')) {
+            Configure::load('app_mercure');
+        }
     }
 
     /**