Passing kwargs to Schema

[Philipp-g]

Since #482 Cornice only accepts a Marshmallow Schema instead of a Schema instance. So I was wondering if it is possible to pass arguments to the schema instantiation.

I just glanced through the code and this suggests that it is not possible right now.

cornice/cornice/validators/_marshmallow.py

Lines 188 to 192 in 0359fdd

	def _instantiate_schema(schema):
	if not inspect.isclass(schema):
	raise ValueError('You need to pass Marshmallow class instead '
	'of schema instance')
	return schema()

[leplatrem]

ping @ergo :)