Click REALLY INSISTS you provide on/off switches for your options.
You can use is_flag, but then the presence of the option changes based
on the default value.
Which makes sense, really:
@option("-f", "foobar", is_flag=True, default=False)
you would expect `./cli -f` to have `foobar is True`
whereas with
@option("-f", "foobar", is_flag=True, default=True)
you would expect `./cli -f` to have `foobar is False`, otherwise it's a
no-op
this becomes fun with `default=os.environ.get("SOMETHING")`, because
then the effect of the option CHANGES with a value of environment
variable!
there's two ways around this:
a) don't use defaults, update the flag explicitly, like:
foobar = foobar or os.environ.get("FOOBAR") == "1"
b) forget about is_flag and specify an on/off switch, where the default
value works as intended
since the latter is also technically speaking more correct, i'm doing it
