Add support for multisite-install

Should also support multisite installs.

* I'm not sure if base is actually necessary
* URL is unused for multisite installs, but is still a required attribute because I didn't know what to set as a default. There's nothing really awesome you can set for it. I guess example.com? Or we could just keep it as a required attribute. Thoughts?

I'm happy to make any necessary change, but I think this is pretty solid.