Load template from theme first, fall back to plugin template

washu-people-places-items.php

add_filter( 'template_include', array( self::$instance, 'ppi_load_single_template' ), 99 );
...
function ppi_load_single_template( $template ) {
...
if ( is_singular( 'things' ) ) {
	// look in child or parent themes for template files first
	if ( $theme_template = locate_template( 'single-items.php' ) ) {
		$template = $theme_template;
	} else {
		$template = self::$directories['templates'] . 'single-items.php';
	}
}

return $template;