Create a terraform-export subcommand
Create a terraform-export subcommand

This gives power users to ability to export configuration to terraform
by defining `#terraform_import_commands` and `#to_hcl_json` on their
resource or resource collections.

This ties into #268 which added the ability to mark all resources in a
stack with the "Retain" deletion policy. Doing so then performing
`convection terraform-export` should leave users in a place where they
have a clear exit strategy from CloudFormation jail convection.

rapid7/convection,rapid7/convection