add linting

inc/trait-singleton.php

+<?php
+/**
+ * Trait file for Singletons.
+ *
+ * @package Camo_Image_Proxy
+ */
+
+namespace Camo_Image_Proxy;
+
+/**
+ * Make a class into a singleton.
+ */
+trait Singleton {
+	/**
+	 * Existing instance.
+	 *
+	 * @var object
+	 */
+	protected static $instance;
+
+	/**
+	 * Get class instance.
+	 *
+	 * @return object
+	 */
+	public static function instance() {
+		if ( ! isset( static::$instance ) ) {
+			static::$instance = new static();
+			static::$instance->setup();
+		}
+		return static::$instance;
+	}
+
+	/**
+	 * Setup the singleton.
+	 */
+	public function setup() {
+		// Silence.
+	}
+}