Add import/export of ABC files.
All: Add support for import/export of multi-score files <-> music21 Opus
Note: MEI supports three different ways to store multiple scores in an
MEI file: multiple <mei> elements (in <meiCorpus>), multiple <mdiv>
elements, and multiple <music> elements. We can read/write all three.
By default we write multiple <mdiv> elements, but you can ask for the
other choices, by specifying multipleScoresHostTag='music' or 'mei' on
the write() call.
Note: music21's myOpus.write() will generally write multiple files, so
if you want a multi-score output file, call music21.stream.write(myOpus)
instead.
All import/export: Add support for PedalMarks.
Humdrum/MEI import/export: Support styled lyrics
Humdrum/MEI import/export: Better color support (notes, lyrics, etc)
Humdrum import: stem direction fixes
