let CLI accept input from STDIN

[garettmd]

If no argument is supplied (i.e. template_path) then cf2tf will accept input from STDIN. This lets you do something along the lines of the below for loop to easily turn a large stack of CloudFormation stacks into TF code:

 for STACK_NAME in vpc-dev ec2-dev vpc-prod ec2-prod ; do aws cloudformation get-template --stack-name ${STACK_NAME} --query 'TemplateBody' --output text >> ${STACK_NAME}.yml | cf2tf >> ${STACK_NAME}.tf; done