rename __slots__ to make dump.Options extendable

Hi cortesi,

would you be okay with changing the __slots__ attribute to _slots for dump.Options? I would really appreciate this to stay completly in sync with your mitmproxy repo.
The problem with __slots__ is that it makes extending Options with custom attributes impossible. While this is not an issue per se in mitmproxy, it gets bad when you're going to extend libmproxy (as I do with HoneyProxy).