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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return boto3.resource('s3',
        endpoint_url=config.TEST_S3_URL,
        config=boto3.session.Config(
            s3={'addressing_style': 'path'}),
        verify=False)
def extract_region_from_auth_header(headers):
    auth = headers.get('Authorization') or ''
    region = re.sub(r'.*Credential=[^/]+/[^/]+/([^/]+)/.*', r'\1', auth)
    region = region or get_region()
    return region
def extract_region_from_arn(arn):
    parts = arn.split(':')
    return parts[3] if len(parts) > 1 else None
def get_account_id(account_id=None, env=None):
    if account_id:
        return account_id
    env = get_environment(env)
    if is_local_env(env):
        return os.environ['TEST_AWS_ACCOUNT_ID']
    raise Exception('Unable to determine AWS account ID (%s, %s)' % (account_id, env))
def role_arn(role_name, account_id=None, env=None):
    if not role_name:
        return role_name
    if role_name.startswith('arn:aws:iam::'):
        return role_name
    env = get_environment(env)
    account_id = get_account_id(account_id, env=env)
    return 'arn:aws:iam::%s:role/%s' % (account_id, role_name)
def iam_resource_arn(resource, role=None, env=None):