Support exporting output values for cross-stack refrences

[rogerfc]

I have not been able to export the output values of a template, as described in http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/outputs-section-structure.html

I aim to get an ouptut section similar to

"Outputs" : {
  "Logical ID" : {
    "Description" : "Information about the value",
    "Value" : "Value to return",
    "Export" : {
      "Name" : "Value to export"
    }
  }
}

Looking at the code, it does not look possible.

Please add this option so we can make cross-stack references.

[eric-herve]

I propose a pull request to fix missing export.