Support **kwargs in capture/acapture?

[njsmith]

I left this out of the original version of this code because in general in trio this is a complicated issue (python-trio/trio#470), and I figured it was better to be consistent with other trio functions like trio.run, where we need to reserve kwargs for controlling the parent function. But now that it's a separate library, that doesn't really apply. And capture and acapture are obviously complete as they stand; there's no way we'll ever want to add any other arguments to them.

[RazerM]

Agreed, I never considered this when I turned the methods into functions