Allow overriding useFetch's url through run's init

[ghengeveld]

In #75 we added support to change the init value of fetch when using run with useFetch. Unfortunately the url prop cannot be overruled this way because it's not part of the init object (and can't be).

I think the pragmatic approach here is to support a resource prop on run's init object, use that if present and omit it from the actual object we pass to fetch. Alternatively we can add support for a resource argument to run. I'd prefer that to be the first argument, but it would be a breaking change.