Feature request: Support unified Change Sets for nested stacks

### Describe your idea/feature/enhancement
The AWS CLI now supports generating a unified change set for nested stacks, as shown in [the docs](https://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/change-sets-for-nested-stacks.html#change-sets-for-nested-stacks-cli):

```bash
aws cloudformation create-change-set --include-nested-stacks
```

This makes it much easier to tell what is going to happen to your resources, making working with nested stacks more pleasant.

It would be great if the SAM CLI could support a similar configuration point in the `sam deploy` command.

### Proposal

Presumably there's a way to pass the `include-nested-stacks` option to Cloudformation when creating change sets programatically. `sam deploy` could receive a similar configuration option and pass it to Cloudformation.

Things to consider:
1. Will this require any updates to the [SAM Spec](https://github.com/awslabs/serverless-application-model)? No - this wouldn't have any impact on the Serverless resources.

### Additional Details