Add option to observe horizontal intersection

[davidisaaclee]

Thanks for this library – it's a great alternative to other virtualization methods.

I wanted to use this with a horizontal list, but the code assumes vertical scroll. I made these changes for my own use – maybe they'd be useful generally.

Some caveats:

• This PR is a breaking change (replaces defaultHeight with defaultSize). This could be made backwards compatible by accepting defaultHeight as well, and dynamically figuring out which prop to use.
• I think it'd be useful to extend this further so that users don't need to choose vertical or horizontal: allow observing both vertical and horizontal, and allow user to omit the axis they don't care about. (I think that approach would avoid a breaking change.) I might be interested in making this change if I personally need it in the future.