Conditional arguments

[mottosso]

E.g. when qargparse.Boolean("showOptions") is toggled, other arguments should either (1) toggle between enabled/disabled states or (2) hide/show.

# E.g.
show = partser.add_argument("showOptions")
age = parser.add_argument("age", default=50, when=show is True)
height = parser.add_argument("height", default=1.90, when=show is True, when_mode=qargparse.Hide)