Allow relative links, add a config for disallowing

DependencyInjection/Configuration.php

@@ -0,0 +1,23 @@
+<?php
+
+namespace Northern\MarkdownBundle\DependencyInjection;
+
+use Symfony\Component\Config\Definition\Builder\TreeBuilder;
+use Symfony\Component\Config\Definition\ConfigurationInterface;
+
+class Configuration implements ConfigurationInterface
+{
+    public function getConfigTreeBuilder(): TreeBuilder
+    {
+        $treeBuilder = new TreeBuilder('northern_markdown');
+        $treeBuilder->getRootNode()
+                    ->children()
+                        ->booleanNode('allow_relative_links')
+                            ->info('Allow relative links when sanitizing.')
+                            ->defaultValue(true)
+                        ->end()
+                    ->end();
+
+        return $treeBuilder;
+    }
+}

DependencyInjection/NorthernMarkdownExtension.php

@@ -13,5 +13,11 @@ public function load(array $configs, ContainerBuilder $container)
     {
         $loader = new XmlFileLoader($container, new FileLocator(__DIR__ . '/../Resources/config'));
         $loader->load('services.xml');
+
+        $configuration = new Configuration();
+        $config        = $this->processConfiguration($configuration, $configs);
+
+        $maintenanceSubscriberDefinition = $container->getDefinition('northern_markdown.markdown_parser');
+        $maintenanceSubscriberDefinition->setArgument('$allowRelativeLinks', $config['allow_relative_links']);
     }
 }

Service/MarkdownParser.php

@@ -11,13 +11,19 @@ class MarkdownParser implements MarkdownParserInterface
 
     private HtmlSanitizer $sanitizer;
 
-    public function __construct()
-    {
+    public function __construct(
+        bool $allowRelativeLinks
+    ) {
         $this->parsedown = new \Parsedown();
 
         $sanitizerConfig = new HtmlSanitizerConfig();
         $sanitizerConfig = $sanitizerConfig->withMaxInputLength(1_000_000);
         $sanitizerConfig = $sanitizerConfig->allowSafeElements();
+
+        if ($allowRelativeLinks) {
+            $sanitizerConfig = $sanitizerConfig->allowRelativeLinks();
+        }
+
         $sanitizerConfig = $sanitizerConfig->allowAttribute('class', '*');
         $sanitizerConfig = $sanitizerConfig->allowAttribute('style', '*');