Set map zoom based on url parameter if given

Refactoring of https://github.com/ffnord/ffmap-d3/pull/89
nope, it's fine as is. Line 88 takes care, that getMaxZoom case is covered. Anyway, I agree this is a little hard to read and it takes a little time to understand what is happening. I tried to clarify this a little, be rewriting the logic, but gave up after some time. I'd leave this as is ;)