Add --offline capability

[matthiasbeyer]

I want to run the tool without internet access (inside a nix build). But right now it does not seem to be possible.

Could a feature for this be added?

[apljungquist]

I too would like this (I actually want reproducible results, but the inconsistent results seem to go away when I set no-clearly-defined, so I'm guessing an offline mode would help me as well).