"load_only" fields

[sloria]

Possible feature: add a Field parameter that would make the field active for validation/deserialization but not appear in serialization output.

This came up in a discussion here: marshmallow-code/flask-marshmallow#5 (comment) (though we used the term "write-only", which is a misnomer).

Related to #61 .

EDIT: Fix URL to comment.

[sloria]

Added in #131

[Lacrymology]

is there a way to have dump_only fields throw an error if they're passed on to load?

[lafrech]

@Lacrymology, this could be a sub-use case of the check_unknown_fields example: http://marshmallow.readthedocs.io/en/latest/extending.html#validating-original-input-data

It seems the authors don't see this as a feature that should be in the core, available through a parameter, but leave this to user code (#524).