Add validation hook to route definition

[pdiazvargas]

What

I want to have the ability to validate a request before my view gets executed. The validator's only responsibility would be that of making sure that the payload of the request is valid. Also, the validator should have access to the route, headers...

@app.route('/users', methods=['POST'], validator=UserValidator())
def create_users():
    created_users = get_created_users(app.request.json_body)
    return [u.to_dict() for u in created_users]