Ability to reference outputs across regions

## 1. Title
Ability to reference outputs of a stack in another region

## 2. Scope of request
In our environments we have created cloudfront distributions in a european region via cloudformation. We are now creating a lambda@edge function for use with one of these distributions. Because lambda@edge can only be created in us-east-1, we are now in the situation where we have to hardcode the ARN, or retrieve it via the CLI and pass it in as a parameter, for use in our cloudfront stack in the european region. For simplicity we would much rather be able to use a cross stack reference that specifies the region the stack exists in.

## 3. Expected behavior

I should be able to optionally specify a region in my cross stack reference, to pull in an output value from a cloudformation stack deployed in a different region.

Start by reviewing how CloudFormation cross-stack references handle stack outputs and region boundaries, using the CloudFront and Lambda@Edge scenario described in the issue. Done means an optional region can identify an output from a stack deployed in another region without hardcoding its ARN or retrieving it through the CLI.

Written by the indexing model from the issue text.