check_plain strips html in message

Line 71 of the module currently uses:
drupal_set_message(check_plain($logout_message));

This results in any html markup (eg, links) being rendered as plaintext.

This can be fixed by using filter_xss instead:
drupal_set_message(filter_xss($logout_message));

This is a useful ability for messages like:

You have logged out successfully. Log in again?

You have logged out successfully. <a href="/user">Log in again?</a>