Allow `doorstop import` to add new items.

Currently, only rows with UIDs will be processed.

-----

TODO:

- [ ] on export, add new rows with "(auto)" as the UID
- [ ] update `doorstop.core.importer` to accept empty rows (with at least `text`)
- [ ] on empty rows, create a new item (assigned the next UID)
This code should probably be passed into core (dependency injection).