Added --stack-name option to cfn-describe-stacks

This is an edge case, but if the stack name is empty (user left it out accidentally), the command will return all stacks which is not what we want. Adding the --stack-name explicit parameter name will prevent this error from happening.
Fixed the names of the maps to match the names expected in the example script
Minor: renamed external map files
Retrieve only the specified map from external map file
Merge pull request #13 from csgyuricza-bv/master

Fixes the incorrect map interpretation from external files in JSON, Ruby and YAML formats