Address in registration form

[stuartpb]

Sometimes a site, like a registrar like Namesilo (see #181), will require a full mailing address and/or contact info on registration. If we're going to move to an exhaustive "everything that's required on registration is mentioned in registration.form" model, it'll have to handle these.

(On that note, maybe it's worth noting this in the interim via notes?)

Unlike #182, this one is a bit more complicated and liable to cork around with #6, so I'm putting this on future instead of v0.1.0.