Problems creating HTML formatted emails in Drupal 8

Hello,
I need help for creating HTML formatted emails in Drupal 8.0.5.

I want to make registration confirmation email in HTML format, to support HTML tags and images in "Account settings" configuration page ("admin/config/people/accounts" ).

I have tried with adding HTML tags inside "Account settings" configuration page . But while triggering the email , it is filtering and converting HTML to plain text .
Also tried altering hook_mail , and converting "['headers']['Content-Type'] = 'text/html"

function custom_module_mail_alter(&$message) {
  switch ($message['key']) {
    case 'register_no_approval_required':
	      $message['headers']['Content-Type'] = 'text/html; charset=UTF-8; format=flowed; delsp=yes';
      break;	  
  }	   
}

Still email is sending as plain text instead of HTML .

Is any one have issue fix for this . Appreciate your help.

Thank you .
Rabith Kuniyil.