docs(luks): add an example using keyFile options

[rogarb]

This adds keyFileOffset and keyFileSize options in example/complex.nix. It follows the discussion in #301.

I thought that adding a new example file for these options was a lot of noise for little result.

@Lassulus if you meant something else with "a new test", please put me on tracks, as I quickly checked the tests folder and couldn't think about something relevant I could add there.

[rogarb]

Mmm it seems that I asked too many bytes to read from the keyfile. I should have checked lib/tests.nix more thoroughly. It should work now.