Add support for various modal presentation styles

[benkraus]

This adds support for configuring the modal presentation style on a view presented modally in iOS. Full screen is the system default, which is cool, but not helpful when you want to show something as a form sheet in horizontally regular environments (iPhone plus sized in landscape, iPad), or as a popover.

This allows modalPresentationStyle to be passed as an option when presenting modally.

[gusbicalho]

Would #69 solve your use case?

[benkraus]

No. That only supports overCurrentContext in a very small use case. This allows for popovers, form sheets, etc.

[lelandrichardson]

@benkraus this is great, and I think we should have a way to solve this. We should potentially think about what the equivalent concepts on Android are and see if we can map them in the context of this PR.

@gpeal do you have any thoughts on this?

[benkraus]

Yup, would totally love to see the Android side of this represented as well. The other chunk missing here is the preferredContentSize, which would allow setting a different size other than the default for when showing a view as a popover.

[gpeal]

@lelandrichardson @benkraus There isn't a direct analogy to these values on Android but reading through the docs, it looks like a fair number of them could be reproduced which is good. Seems like a useful feature if you're only supporting iOS though.