allow npe2 validate to notify on extra fields

[nclack]

Description

Once #237 is merged, the validation performed when npe2 reads a manifest will be less strict. Extra fields will be ignored. This is desirable because it's nice to prefer compatibility when reading manifests.

Strict validation is desirable when a developer is creating a manifest. It's desirable to see extra fields, including, e.g. spelling mistakes.

Proposed new feature: Add a flag to the npe2 validate command that controls strictness - optionally turning on/off detection of extra fields.