cli: --offline means fully offline

[woodruffw]

With this change, --offline also disables TUF
repository updates and not just online log lookups.

Needs unit and integration tests; working on that now.

Closes #483. Closes #376.

CC @sethmlarson

[woodruffw]

Bundle verification in the offline tests passes:

test/unit/verify/test_verifier.py::test_verifier_bundle_offline[bundle.txt] PASSED [ 95%]
test/unit/verify/test_verifier.py::test_verifier_bundle_offline[bundle_v3.txt] PASSED [ 96%]
test/unit/verify/test_verifier.py::test_verifier_bundle_offline[bundle_v3_alt.txt] PASSED [ 96%]

I think the last remaining thing to do here is figure out an appropriate log/warning message to raise: passing --offline now means that the user doesn't get the security benefits of checking the TUF repo for revocations. The security impact of this may be marginal in practice (especially if the local trust root was updated recently anyways), but we should still notify the user that their posture is slightly different.