Refactored node type register command to be more flexible re. loading

This PR:

- Allow multiple files to be specified
- Allow folders to be specified

Its a BC break in that it forces at least one `target` option to be secified, whereas before there was a required argument for a single CND file.

This PR also keeps in mind support for YAML files instead of CND (I am currently working on a YAML importer + dumper).