Chord symbol extensions are not parsed correcly

[FelipeDefensor]

E.g. G7#9, C7b13, C7(11, 13)

As @azfoo mentioned in #349 , music21.harmony.ChordStepModifications might give us the informatioin we need to extend the vanilla music21 parsing.

[FelipeDefensor]

I have also considered:

• Using a another library to parse chord symbols: There seems to be none in Python. There is the chord_symbols JS library, but I would rather not use QtWebEngine any more than necessary. We could use js2py to run JS code in Python, but that sounds error-prone.
• Writing a chord symbol parsing library in Python: possible, but a lot of work.