Allow applying markers via case(..., marks=...)

[smheidrich]

My suggestion for how to implement #7 that I mentioned at the end of that issue. Adds a new marks parameter to Parametrization.case(), in analogy to pytest's own pytest.param(..., marks=...).

As I wrote in #7, this solution has the drawback of introducing a new reserved keyword argument for case(), but I still find it so much more ergonomic than the alternatives that to me it would be worth it. Might need to wait until the next major release though because it should be considered a breaking change...

Thoughts?

[beebikun]

That would be super helpful! I was just about to suggest the same changes.

But I think, maybe _marks is a better name, so there were less chances of names clashing?