Pass arguments to the default command

[WhyNotHugo]

Pass arguments to the default command so that stuff like todo --startable work.

Not sure if I want to merge this as-is, or actually contribute to click-default-group and make the default configurable. TBH, I with that I could make the group as a class, instead of a function with a bunch of decorators, but I don't know how much this is supported.

Fixes #237