Expose height prop for modal component

[btroop]

I would like all of my modals to be the same height in proprotion to the screen, rather than adjusting to the size of the content.

I realize that I can do that with the styles api, but it would be convenient to have the "height" prop exposed for the component, just like the "size" prop...it seems logical that if width is controllable without using the styles api, the height should be too.