filter amp_template_dir added

this filter might help to use completely different template files.

includes/class-amp-post-template.php

@@ -26,7 +26,7 @@ class AMP_Post_Template {
 	private $data;
 
 	public function __construct( $post_id ) {
-		$this->template_dir = AMP__DIR__ . '/templates';
+		$this->template_dir = apply_filters( 'amp_template_dir', AMP__DIR__ . '/templates' );
 
 		$this->ID = $post_id;
 		$this->post = get_post( $post_id );