- Converted `Item.issues()` to a property and added `Item.get_issues()`.
- Added '--level' option to `doorstop add` to force an item level.
- Added warnings for duplicate item levels in a document.
- Added warnings for skipped item levels in a document.
- Renamed `Item` methods: add_link -> link, remove_link -> unlink, valid -> validate.
- Renamed `Document` methods: add -> add_item, remove -> remove_item, valid -> validate.
- Renamed `Tree` methods: new -> new_document, add -> add_item, remove -> remove_item, link -> link_items, unlink -> unlink_items, edit -> edit_item, valid -> validate.
- Added `doorstop.importer` functions to add exiting documents and items.