Allow HTML based welcome messages
I needed a HTML based welcome message, so I added this to mail_plugin.inc.php
I hope this is the correct approach.
In addition to /conf-custom/mail/welcome_email_DOMAINNAME.txt and /conf-custom/mail/welcome_email_LANGUAGE.txt one may place a /conf-custom/mail/welcome_email_DOMAINNAME.html or /conf-custom/mail/welcome_email_LANGUAGE.html file to allow HTML based welcome messages.