Add `--no-download` flag for easier/better packaging

This should address (partially) issue #76 (the pre-download step is the remaining part).

I've modified the existing test, although I'm not sure if I've done it right, but cabal test passes (both with and without `--no-net`).
Yes, I think not touching the URLs is better. If you still want to add a single backup check then I'd do it inside wget (and modify that to take `CmdLine`) - your choice whether you want to bother or not though - at the moment `wget` is only called from the `downloads` rule, and I don't envisage that changing.