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.
Also @jipiboily ``site_id`` should be valid on it's own, but ``custom_url`` should require ``site_id``...I don't think this is the logic you have.