add new site-id and custom-url options

This will add two new options, `site-id` and `custom-url`. Used in tandem, you can have RF runs over some ad-hoc staging or QA environments.

This is a WIP and just adds the options and doc, it does nothing yet.
Yeah, that's one thing I thought too when I was getting back home...but at the same time, a site name could be duplicated so that's not enough. That said, we could add in there, "get in touch with us for help on getting that id"?