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`).
It is a bit excessive, indeed. What I wanted to do is to ensure that an accidental refactoring won't remove a single test for no-download. But that was before I added the test, so maybe relying on the test is enough?

If you'd prefer to not touch the URLs, I'll update the patch.