Pychord

- Add modes to `RELATIVE_KEY_DICT`.
- Add `diatonic` option for `from_note_index`.
- Contributor: hejops

- Add `add4`, `add11` and `add13` quality variations.
- Contributor: philipmat

- Fix `dim6` to `dim7`.
- Add `M7+11` and `m7+5` qualities.

- Add an example to create a MIDI file.
- Add `QualityManager` class to overwrite default qualities.
- Do not import `QUALITY_DICT` from modules other than quality.
- Fix `sus` quality.

- Add `m7b9b5` quality.

- Add Chord.components_with_pitch method.