Add theme directory default shortcode

Looking to add a default placeholder for the theme directory for future versions.
Example:

<img src="{{template_directory_uri}}/images/{{backgroundimg}}.png">
means we can do
[shortcode backgroundimg="child.jpg"]
without hardcoding a URL, making it adapt to future themes

# diff -urN includes/placeholders.{bak,php}
--- includes/placeholders.bak   2017-10-02 11:37:10.632457601 -0400
+++ includes/placeholders.php   2017-10-02 11:43:31.108480218 -0400
@@ -32,6 +32,11 @@
             return \add_query_arg(array(), $wp->request);
         }
 
+       // Returns the theme/template directory
+       if ($name == 'template_directory_uri') {
+               return \get_template_directory_uri(); 
+       }
+
         /**
          * PHP
          */