Add option to fail on unexpected fields

[maxgrenderjones]

Per the docs:

Normally, unspecified field names are ignored by the validator. If you would like access to the original, raw input (e.g. to fail validation if an unknown field name is sent), add pass_original=True to your call to validates_schema.

If I'm trying to use marshmallow to do validation that the input is what I expect and I want to fail if I get unexpected data, is there really no way to have it fail without writing your own validator? (Apologies if I'm just missing something obvious in the docs - this seems very surprising as someone new to marshmallow).

If not, feature request: please could you add a fail_on_unexpected flag to accompany strict (which would have to default to False for backwards compatibility reasons? (I guess strict is some combination of hypothetical fail_on_missing and validate keywords?)

[sloria]

Let's continue discussion of this in #524