Adding the ability to manage AuthExternal parameter.

Hey guys.

This patch save my day when I tried to authenticated against unixgroup users and I was unable to manage this parameter trough the original module, so I've created this little patch.

I hope this could be helpful for you.

Cheers,
Alberto

Hey,

Wow, sorry. No reason. I just placed at the bottom of auth_* params. I don't figured out that it is on alphabetic order. I'll fix it and make a new pull request.