Label Chord N-Grams with Root-and-Figured-Bass

It was so easy---thanks, music21!

Closes #209
Enabled Chord N-Gram Annotation on a Score

It was just lying there, begging to be added!
Update IntervalNGram.canonical() and Its Tests

The canonical() method now accepts two arguments for interval size and
quality.
Option to Ignore Interval Inversion and Int. N-Gram Voice Crossing

New checkbox in the GUI, and improved logic elsewhere. So fun.

Closes #199