Add new combo and combo skip to HitObject

[OliBomby]

Seemed like this was missing. I need new combos for my project.

Spinners dont parse the combo skip value because this mimics osu! stable behaviour.
Hold notes also dont parse the combo information.

Closes #112

[tybug]

I've made some changes here. Before, this pull included breaking changes: from slider import Circle; c = Circle(...) is part of the public api and any new parameters should have a default value (unless we want to bump the major version, which I don't particularly want to right now).

I've also gone ahead and parsed the combo skip for every hitobject. Even if combo skip values are unused for certain hitobjs by osu stable, as long as they are present in the type information of the beatmap I think it is more correct for us to parse them according to spec and provide them. A consumer can then decide how to use the combo skip number in the context of different hitobjs.

I haven't tested this besides making sure code tests pass, so probably worth a double check to ensure I didn't break anything.

[tybug]

thanks for the HoldNote fix 👍

this looks good to me