- register all import/export handlers via setup.py and config
- use "handler key" lookup for all import/export commands
- fix logic bugs w/ CORE -> Trainwreck importer
i.e. `rattail_corepos.corepos.importing.db` is now the place for direct DB
importers, and `rattail_corepos.corepos.importing` will become the place for
proper API-based importers