Implement --at-least-one-of

[kornelski]

Skips powersets that don't contain any of the specified features. Can be used to test cfg-if chains of features where the last else is a compile error.

Fixes #182

[taiki-e]

Thanks!

Could you add a test for this? (I think we can add a test that processes the input in the same way as src/main.rs#L244-L258 and checks the result, to the tests module in features.rs.)

[kornelski]

Change in MSRV is unrelated. Could be fixed with cargo update -p ctrlc --precise 3.1.4, but I don't see how this could work without lockfile and with your custom msrv workflow.

[taiki-e]

I fixed the MSRV issue on the main branch, could you rebase?

[taiki-e]

Published in 0.6.5. Thanks @kornelski!