Add configuration flag allowing the use of a custom b2d ISO source URL

Until now we have been using a heavily patched fork of the now-retired bash-based boot2docker script, but all we need on the script-side (apart from the ability to use a custom ISO) is now in the go port, so I'd like to move on and benefit from the all-in-one DMG installer on OS X.

I think there will always be a need for custom ISOs. We am using one currently in order to a company proxy for the docker engine and auto-create a larger swap partition at the first boot. These are of course things that could be made configurable, but is it really worth it? There will be always specific/exotic needs adding up over time, so this allows advanced users to do their own thing.
If we turn around that PR as discussed just above so that the flag is `--iso-url` supporting a 1- or 2-steps process rather than a 1-step override, how we would qualify an ISO as "custom" (and therefore give it another `B2D.ISO` default)? Would matching `github.com/repos/boot2docker/boot2docker/releases` be enough to tell it's "official"? It seems a little brittle. Plus, that introduces a coupling between the remote/local ISO configuration flags. Did you have any specific use-cases/flows in mind?