Hi

I have issues creating sns subscriptions through Cloudformation. If I attempt to include a `AWS::SNS::Subscription` resource in a Cloudformation template and run `create-stack` then I get a 500 Internal Server error. The full template:

```
AWSTemplateFormatVersion: '2010-09-09'
Description: 'Test'
Resources:
  MySnsTopic:
    Type: AWS::SNS::Topic
    Properties:
      TopicName: MySnsTopic
  MySnsTopicSubscription:
    Type: AWS::SNS::Subscription
    Properties:
      Protocol: sqs
      TopicArn: !Ref MySnsTopic
      Endpoint: !GetAtt
        - MySqsQueue
        - QueueArn
  MySqsQueue:
    Type: AWS::SQS::Queue
    Properties:
      QueueName: MySqsQueue
```

If I replace the references with hard-coded arns like this:

```
TopicArn: arn:aws:sns:eu-west-2:123456789012:MySnsTopic
Endpoint: arn:aws:sqs:elasticmq:000000000000:MySqsQueue
```

Then I get no errors but I get a warning:

`No moto Cloudformation support for AWS::SNS::Subscription`.

I can get around this issue by creating the subscription when creating the topic but the issue there is that I cannot add any attributes and in my case I need `RawMessageDelivery` set to true. So my questions are:

1. Is there no support to create `AWS::SNS::Subscription` resources through Cloudformation or am I doing something wrong?
2. If support is not there can I request it as a new feature?
3. Any suggestions on how I can work around this? I know I could do this through a script at startup but would like to not have any additional script steps other than deploying the Cloudformation stack.

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def update_resource(resource_id, resources, stack_name):
    resource = resources[resource_id]
    resource_type = get_resource_type(resource)
    if resource_type not in UPDATEABLE_RESOURCES:
        LOG.warning('Unable to update resource type "%s", id "%s"' % (resource_type, resource_id))
        return
    props = resource['Properties']
    if resource_type == 'Lambda::Function':
        client = aws_stack.connect_to_service('lambda')
        keys = ('FunctionName', 'Role', 'Handler', 'Description', 'Timeout', 'MemorySize', 'Environment', 'Runtime')
        update_props = dict([(k, props[k]) for k in keys if k in props])
        return client.update_function_configuration(**update_props)
    if resource_type == 'ApiGateway::Method':
        client = aws_stack.connect_to_service('apigateway')
        integration = props.get('Integration')
        
        kwargs = {
            'restApiId': props['RestApiId'],
            'resourceId': props['ResourceId'],
            'httpMethod': props['HttpMethod'],
            'requestParameters': props.get('RequestParameters')
        }
        if integration:
            kwargs['type'] = integration['Type']
            kwargs['integrationHttpMethod'] = integration.get('IntegrationHttpMethod')
            kwargs['uri'] = integration.get('Uri')
            return client.put_integration(**kwargs)
        kwargs['authorizationType'] = props.get('AuthorizationType')
        return client.put_method(**kwargs)
def convert_data_types(func_details, params):