Ability to customize style of ExNavigationStack and ExNavigationStackItem

[jeanregisser]

Hi there,

First of all great lib! It really makes it so much easier to use NavigationExperimental ;-)

When playing with the example app I wanted to customize the white background that appears when opening a modal.

See the following:

The only way I found was to manually edit https://github.com/exponentjs/ex-navigation/blob/79a7a5b87007bffe35b0947ac23996b20be9d6e1/src/ExNavigationStack.js#L693

Though I know the floatVertical anim is going to lose its scale soon (#37), I really like it for modals and I'll keep it internally (or maybe you can leave it under a new name?).

The other thing I wanted to style is the shadow of ExNavigationStackItem (also visible in the screenshot above).
I haven't seen a way to pass down a style for those items yet.

I'm happy to send a PR to allow styling those elements if you want.

Let me know what you think.