Custom group subdomain from database

Hello, first of all thanks for this great module!

I wanted to use Subdomains to provide a custom subdomain for my site's organic groups. However, I didn't want it to be tied to the groupname or id, I needed the subdomain to be completely independent. So I hacked the module to suit my needs.

With this small patch, when configured in OG mode, the module, instead of deriving the subdomain from the title, will look in a new "subdomain" table to retrieve the subdomain for the current group.

I'm submitting this patch for your review: maybe I'm breaking something else by doing this or introducing some kind of security risk... Otherwise perhaps this will be useful to other users of this fine module.

Of course, it would be great if this functionality was integrated...