refactor: add php_codesniffer to define castopod's coding style based on psr-1

- add .editorconfig file
- format all files to comply with castopod's coding style
- switch parsedown dependency with commonmark library to better follow commonmark spec for markdown
- add prettier command to format all project files at once

closes #16

app/Models/AnalyticsWebsiteByRefererModel.php

@@ -1,4 +1,5 @@
 <?php
+
 /**
  * Class AnalyticsWebsiteByRefererModel
  * Model for analytics_website_by_referer table in database
@@ -6,6 +7,7 @@
  * @license    https://www.gnu.org/licenses/agpl-3.0.en.html AGPL3
  * @link       https://castopod.org/
  */
+
 namespace App\Models;
 
 use CodeIgniter\Model;
@@ -17,7 +19,7 @@ class AnalyticsWebsiteByRefererModel extends Model
 
     protected $allowedFields = [];
 
-    protected $returnType = 'App\Entities\AnalyticsWebsiteByReferer';
+    protected $returnType = \App\Entities\AnalyticsWebsiteByReferer::class;
     protected $useSoftDeletes = false;
 
     protected $useTimestamps = false;