AWS::CloudFormation::StackInstance - New Resource

1. Title -> AWS::CloudFormation::StackInstance
2. Scope of request -> A StackInstance resource that can be used to add Stacks to a StackSet
3. Expected behavior -> It takes a !Ref to a StackSet (see #102), an AccountId and a Region. One StackInstance should be exactly one account+region.
5. Links to existing API doc (optional) -> https://docs.aws.amazon.com/AWSCloudFormation/latest/APIReference/API_CreateStackInstances.html
6. Category tag (optional) -> Management & Governance
7. Any additional context (optional)
The API allows creating mulitple stack instances at the same time. This may be a possible way to do it, but I think that having one Resource per Instance makes more sense in a CloudFormation template.