Allow disabling mdarea via admin setting (#27)

extensions/markdown/migrations/2021_03_25_000000_default_settings.php

@@ -0,0 +1,14 @@
+<?php
+
+/*
+ * This file is part of Flarum.
+ *
+ * For detailed copyright and license information, please view the
+ * LICENSE file that was distributed with this source code.
+ */
+
+use Flarum\Database\Migration;
+
+return Migration::addSettings([
+    'flarum-markdown.mdarea' => true
+]);