Warn on unknown options

It would be very good if we could output a warning when passing in any option that we don't recognise.

We can't start throwing just yet since that needs to be in the next major bump, but that is the behaviour that I would most prefer.

Start by tracing how Multer parses and validates its options, then inspect any existing tests for option handling. Define the warning behavior for unrecognized options while preserving the current non-throwing behavior; done means unknown options produce warnings and recognized options continue to work.

Written by the indexing model from the issue text.