- Added `PREPEND` settings prefix because sometimes items need to be added to
the beginning of a list, not the end (e.g., `MIDDLEWARE_CLASSES` is a good
example of this).
- Renamed `EXTRA` settings prefix to `APPEND`; this is more precise and mirrors
`PREPEND`.
- Keep settings in order when reading from file. When reading INI files, take
care to order items from the `[DEFAULT]` section before the items in the
specified section. This change *should* allow the `SWAP` prefix to be
removed (but we'll keep it for now).
- In derived settings files, don't require an empty section to be present if
the section is present in a base settings file. It's nice to not have to add
empty sections to a derived settings file for sections that don't have any
overridden options.