no default arg kills execution on mac High Sierra

[zenfish]

Line 43 in c2m.py currently reads -

parser.add_argument('-O', '--offset', type=float, help='Set the amount to offset each chord, in ticks. (default 0.0)')

Adding a default (like almost all the others in the arg parsing section) makes line 103 not crash and burn when you add 'offset' to some other vars.