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 '%s:%s' % (self.region, self.prefix)
PREDEFINED_ENVIRONMENTS = {
    ENV_DEV: Environment(region=REGION_LOCAL, prefix=ENV_DEV)
}
def get_environment(env=None, region_name=None):
    """
    Return an Environment object based on the input arguments.
    Parameter `env` can be either of:
        * None (or empty), in which case the rules below are applied to (env = os.environ['ENV'] or ENV_DEV)
        * an Environment object (then this object is returned)
        * a string '<region>:<name>', which corresponds to Environment(region='<region>', prefix='<prefix>')
        * the predefined string 'dev' (ENV_DEV), which implies Environment(region='local', prefix='dev')
        * a string '<name>', which implies Environment(region=DEFAULT_REGION, prefix='<name>')
    Additionally, parameter `region_name` can be used to override DEFAULT_REGION.
    """
    if not env:
        if 'ENV' in os.environ:
            env = os.environ['ENV']
        else:
            env = ENV_DEV
    elif not is_string(env) and not isinstance(env, Environment):
        raise Exception('Invalid environment: %s' % env)
    if is_string(env):
        env = Environment.from_string(env)
    if region_name:
        env.region = region_name
    if not env.region:
        raise Exception('Invalid region in environment: "%s"' % env)
    return env
def is_local_env(env):