- Implements shortcodes

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

lib/Newsletter/Shortcodes/Shortcodes.php

@@ -0,0 +1,53 @@
+<?php
+namespace MailPoet\Newsletter\Shortcodes;
+
+class Shortcodes {
+  public $rendered_newsletter;
+  public $newsletter;
+  public $subscriber;
+
+  function __construct(
+    $rendered_newsletter,
+    $newsletter = false,
+    $subscriber = false) {
+    $this->rendered_newsletter = $rendered_newsletter;
+    $this->newsletter = $newsletter;
+    $this->subscriber = $subscriber;
+  }
+
+  function extract() {
+    preg_match_all('/\[(?:\w+):.*?\]/', $this->rendered_newsletter, $shortcodes);
+    return array_unique($shortcodes[0]);
+  }
+
+  function process($shortcodes) {
+    $processed_shortcodes = array_map(
+      function ($shortcode) {
+        // TODO: discuss renaming "global". It is a reserved name in PHP.
+        if($shortcode === 'global') $shortcode = 'link';
+        preg_match(
+          '/\[(?P<type>\w+):(?P<action>\w+)(?:.*?default:(?P<default>.*?))?\]/',
+          $shortcode,
+          $shortcode_details
+        );
+        $shortcode_class =
+          __NAMESPACE__ . '\\Categories\\' . ucfirst($shortcode_details['type']);
+        if(!class_exists($shortcode_class)) return false;
+        return $shortcode_class::process(
+          $shortcode_details['action'],
+          isset($shortcode_details['default'])
+            ? $shortcode_details['default'] : false,
+          $this->newsletter,
+          $this->subscriber
+        );
+      }, $shortcodes);
+    return array_filter($processed_shortcodes);
+  }
+
+  function replace() {
+    $shortcodes = $this->extract($this->rendered_newsletter);
+    $processed_shortcodes = $this->process($shortcodes);
+    $shortcodes = array_intersect_key($shortcodes, $processed_shortcodes);
+    return str_replace($shortcodes, $processed_shortcodes, $this->rendered_newsletter);
+  }
+}