added
- `Segment` class, attrs-based
+ has `asdict` method (wrapper around `attrs` function)
+ has class variable `_FIELDS` which is used in any place
where we need to know how to go from `Segment` attributes to rows of
a csv file, e.g. in src/crowsetta/csv.py and in tests
changed
- `Sequence` class is now attrs-based, has factory functions, is itself
just a list of `Segment`s
+ now has `to_dict` method
- `Crowsetta` class is now called `Transcriber`