Fix template loading in template_files method to account for child themes

[actual-saurabh]

In lines https://github.com/yapapaya/live-comment-preview/blob/master/classes/class-live-comment-preview.php#L190-L193
$files = array( 'user' => get_stylesheet_directory() . '/comment-preview.php', 'theme' => get_stylesheet_directory() . '/comments.php', );
only looks for the files in the current theme.

This does not account for a child theme/parent theme relationship. See how WordPress's locate_template function works in https://github.com/WordPress/WordPress/blob/master/wp-includes/template.php#L605-L639

I used it on a child theme and it ended up returning an empty array.