Added support for `Fn::ImportValue`

[nlang]

I needed to reference a resource that is exported by another stack, so I added that functionality. It uses CloudFormations listExports function to substitute exported values from other stacks. I also updated the README.

E.g.

    ...
    environment:
      FOO_ENV_VAR:
        'Fn::ImportValue': ExportNameFromAnotherStack

Merging and publishing a new version would be highly appreciated. Please let me know if I can change or clarify things.