Password confirm field type should have support for additional attributes (e.g. placeholder, etc)

Basically I assigned a 'placeholder' attribute to the password key via a form_alter. But the form_process_password_confirm simply overwrites any attributes by the following:

'#attributes' => array('class' => array('password-field')),

One way around this might be to simply do the following:
'#attributes' => $element['#attributes'] + array('class' => array('password-field')),

But we may want to provide a more robust way of checking so we don't overwrite any classes as well. Here is a very quick patch that obviously is just a starting point for the discussion.