Add the ability to create variables to a variables.tf file based on the configuration

[Justin O'Connor]

A user should be able to create variables for a given resource using a command like:

terraflow variable create --name some_name --type some_type --description some_description --default some_default

--name, --type, and --description, and default should be strings.

If the user wants to try to adopt an existing attribute description or type, they should be able to add some flag like --auto-description or --auto-type