Serialization back to Volpiano (and possibly GABC)

[igneus]

Music loaded using chant21 contains details (word and syllable boundaries) which get lost when serializing back to Volpiano using music21.volpiano.fromStream(). It would be useful to have a custom serialization method taking them in account.

Is this something you would consider adding to chant21 - e.g. by accepting a pull request - or do you consider it out of the library's scope?