- Implements shortcodes

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

lib/Newsletter/Shortcodes/Categories/User.php

@@ -0,0 +1,54 @@
+<?php
+namespace MailPoet\Newsletter\Shortcodes\Categories;
+
+use MailPoet\Models\Subscriber;
+
+require_once(ABSPATH . 'wp-includes/pluggable.php');
+
+class User {
+  /*
+    {
+      text: '<%= __('First Name') %>',
+      shortcode: 'user:firstname | default:reader',
+    },
+    {
+      text: '<%= __('Last Name') %>',
+      shortcode: 'user:lastname | default:reader',
+    },
+    {
+      text: '<%= __('Email Address') %>',
+      shortcode: 'user:email',
+    },
+    {
+      text: '<%= __('Wordpress user display name') %>',
+      shortcode: 'user:displayname | default:member',
+    },
+    {
+      text: '<%= __('Total of subscribers') %>',
+      shortcode: 'user:count',
+    }
+   */
+  static function process($action, $default_value, $newsletter = false, $subscriber) {
+    if(is_object($subscriber)) {
+      $subscriber = $subscriber->asArray();
+    }
+    switch($action) {
+      case 'firstname':
+        return ($subscriber) ? $subscriber['first_name'] : $default_value;
+      case 'lastname':
+        return ($subscriber) ? $subscriber['last_name'] : $default_value;
+      case 'email':
+        return ($subscriber) ? $subscriber['email'] : false;
+      case 'displayname':
+        if($subscriber && $subscriber['wp_user_id']) {
+          $wp_user = get_userdata($subscriber['wp_user_id']);
+          return $wp_user->user_login;
+        };
+        return $default_value;
+      case 'count':
+        return Subscriber::count();
+      default:
+        return false;
+    }
+  }
+}