Allow developer to external access to plugin instance

Instead create new instance by `new Loader()` use new instance container `Simpleshop::getInstance()`.
It makes container to prevent re-create it twice.
Developer still can force create new instance via argument $instanceId (`Simpleshop::getInstance('test')`).

simpleshop-cz.php

@@ -29,4 +29,4 @@
 /**
  * Start plugin
  */
-new Plugin();
+SimpleShop::getInstance();

src/SimpleShop.php

@@ -0,0 +1,34 @@
+<?php
+/**
+ * @package Redbit\SimpleShop\WpPlugin
+ * @license MIT
+ * @copyright 2016-2019 Redbit s.r.o.
+ * @author Redbit s.r.o. <[email protected]>
+ */
+
+namespace Redbit\SimpleShop\WpPlugin;
+
+class SimpleShop {
+	/**
+	 * @var Plugin
+	 */
+	private static $instance = [];
+
+	/**
+	 * @return Plugin
+	 */
+	public static function getInstance() {
+		if ( self::$instance === null ) {
+			self::$instance = self::factory();
+		}
+
+		return self::$instance;
+	}
+
+	/**
+	 * @return Plugin
+	 */
+	protected static function factory() {
+		return new Plugin();
+	}
+}