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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import os
import re
import json
import time
import boto3
import base64
import logging
import six
from localstack import config
from localstack.constants import (
    REGION_LOCAL, LOCALHOST, MOTO_ACCOUNT_ID,
    ENV_DEV, APPLICATION_AMZ_JSON_1_1, APPLICATION_AMZ_JSON_1_0,
    APPLICATION_X_WWW_FORM_URLENCODED, TEST_AWS_ACCOUNT_ID)
from localstack.utils.common import (
    run_safe, to_str, is_string, is_string_or_bytes, make_http_request,
    is_port_open, get_service_protocol)
from localstack.utils.aws.aws_models import KinesisStream
ENV_ACCESS_KEY = 'AWS_ACCESS_KEY_ID'
ENV_SECRET_KEY = 'AWS_SECRET_ACCESS_KEY'
ENV_SESSION_TOKEN = 'AWS_SESSION_TOKEN'
LOG = logging.getLogger(__name__)
LOCAL_REGION = None
CUSTOM_BOTO3_SESSION = None
CREATE_NEW_SESSION_PER_BOTO3_CONNECTION = False
INITIAL_BOTO3_SESSION = None
BOTO_CLIENTS_CACHE = {}
DEFAULT_TIMER_LOOP_SECONDS = 60 * 50
class Environment(object):