Add sub-generator folder references to package.json on sub-generator creation

When creating a sub-generator, yeoman wouldn't add the new folders path to the **files array** `package.json`. This caused the sub-generators not to be available to anyone installing the generator. *json-file-plus* to not lose `package.json`'s formatting

With this PR we add the reference to the sub-generators folder on creation. Also had to add new 

Fixes #61