support for Add9 Add2 chord qualities

[toby11]

this is a brilliant library for creating / getting fingering for chords.

is there a reason why Add9 / Add2 are not supported? I tried adding them in the map in chordClassArray i.e.

{ name: 'Add9', abbrs: ['add9'], intervals: 'P1 M3 P5 M9' },

which seemed ok but the resultant chord is undefined.