Allow for custom overlay styles a-la redbox-react

[khaled]

See https://github.com/commissure/redbox-react#style-object

In my particular case, I just want to tweak zIndex. Shockingly, 9999 isn't high enough, and I don't have control over the host UI.

I know that it's already possible to provide a custom overlay implementation, but that seems like a tall order for such a small tweak.

[glenjamin]

If you can come up with a reasonable API, then i'll take a PR for this.

Alternatively I'd also take a simple PR which makes the zIndex much higher :)