I wonder if we should raise an exception if `_format_name == 'fixed_width_two_line'`. Also, it's not as general as it could be. If I derive anything from `FixedWidthTwoLine` then that new class is not excluded.

On the other hand, this whole affair is only candy to the UI, it only get called with `get_reader`. If I know what I'm doing, I can reset the `start_line` to arbitrary values once I have the reader object. So, for now I think this is good enough (if it passes the tests) but we should keep in mind to find a more pythonic implementation if it gets any more complicated than this in the future.