Add paddingHorizontal, paddingVertical, etc

[dmnd]

React Native has paddingHorizontal, paddingVertical, etc which makes the lack of CSS shorthand less painful.

Aphrodite's style API is already extremely similar to React Native. I'm not sure if that was a design goal, but if so, I think it makes sense to add these extra conveniences to Aphrodite too. It makes Aphrodite less surprising.

See also some previous discussion in #36. That was closed because webapp has a padding mixin. That fine for KA, but I think Aphrodite could benefit by having something built in.