Bugfix for HTML Email Templates plugin

Hi,

Found a small issue in this plugin. On the admin screen, if you click to expand the list of variables that can be used in the templates, there's no entry for {USER_NAME} although this can be used in the templates. The highlighted row in the attached image shows where I've added it to my installation. The edit is in htmlemail.php > list_placeholders() > $placeholder_desc:
'{USER_NAME}' => __( 'Recipient\'s Username', 'htmlemail' ),

Additionally, there are no entries in the list for {BG_IMAGE}, {HEADER_IMAGE}, and {EMAIL_TITLE} although they can be used in the templates. Those can be added in the same fashion/location as the above.

Thanks!