Add prop to enable locking vertically until the image is zoomed past the height.

[JacobJaffe]

Closes #92

This PR aligns the vertical panning behavior with iOS functionality. Currently, zooming is only x-axis locked when the image is at zoom === 1.

This PR introduces a prop, lockVerticalPanUntilExceedsZoom, which makes axis locking occur until the image exceeds the height of the window. This means that small amounts of zoom won't allow vertical panning for the common scenario of horizontal images on a vertical screen.

---

This is an MVP of this functionality. I haven't considered the inverse scenario of images taller than the phone, nor landscape orientation.

This code kinda just gates existing behavior behind some lockY and lockX checks. While that makes it clear in this PR what is happening, it introduces some duplication inside the now-nested conditionals.

Also, the prop name is kinda terrible.

Open to suggestions! Just want to get the ball rolling on this.