This processor function will add extra attributres from LogRecord
objects to the event_dict, and can be useful for making values passed
in the extra parameter of the logging module's log methods pass through
to output as additional properties.

Fixes hynek#209

- fix assert parameter order
- add `add_extra` to `__all__`

This is so ExtraAdder can be configured with what keys it should copy.

- Fix up documentation
- Improve tests