- Implements shortcodes

- Updates newsletter router/sending queue worker to use shortcodes
  replacement class

lib/Newsletter/Shortcodes/Categories/Date.php

@@ -0,0 +1,43 @@
+<?php
+namespace MailPoet\Newsletter\Shortcodes\Categories;
+
+class Date {
+  /*
+    {
+      text: '<%= __('Current day of the month number') %>',
+      shortcode: 'date:d',
+    },
+    {
+      text: '<%= __('Current day of the month in ordinal, ie. 2nd, 3rd, etc.') %>',
+      shortcode: 'date:dordinal',
+    },
+    {
+      text: '<%= __('Full name of current day') %>',
+      shortcode: 'date:dtext',
+    },
+    {
+      text: '<%= __('Current month number') %>',
+      shortcode: 'date:m',
+    },
+    {
+      text: '<%= __('Full name of current month') %>',
+      shortcode: 'date:mtext',
+    },
+    {
+      text: '<%= __('Year') %>',
+      shortcode: 'date:y',
+    }
+   */
+  static function process($action) {
+    $date = new \DateTime('now');
+    $actions = array(
+      'd' => $date->format('d'),
+      'dordinal' => $date->format('dS'),
+      'dtext' => $date->format('D'),
+      'm' => $date->format('m'),
+      'mtext' => $date->format('F'),
+      'y' => $date->format('Y')
+    );
+    return (isset($actions[$action])) ? $actions[$action] : false;
+  }
+}