Allow custom resource constructor args

[multimeric]

For example, this lets you do:

class TestResource(ResourceDetail):
    """ 
    This fake resource always renders a constructor parameter
    """ 
    def __init__(self, *args, **kwargs):
        super().__init__()
        self.constant = kwargs.get('constant')

    def get(self):
        return self.constant

api.route(TestResource, 'resource_kwargs', '/resource_kwargs', resource_kwargs={
    'constant': 'hello!'
})

Basically this is helpful if you ever want to fully customize a resource object.

[coveralls]

Coverage increased (+0.01%) to 90.786% when pulling 3de105d on TMiguelT:resource_init_args into b44bc08 on miLibris:master.