Add support for top/bottom/left/right gutter.

[exogen]

@boygirl I think this does the trick!

This is an alternative to #319 that replaces rowGutter with gutter accepting top, bottom, left, and right properties.

Alternatively, we could keep rowGutter and have gutter={ left, right } + rowGutter={ top, bottom } – thoughts?

Appears to work like a charm, here's gutter={{ left: 20, right: 50, top: 5, bottom: 8 }}:

Fixes FormidableLabs/victory#862.

[boygirl]

@exogen sorry for the delay. I prefer gutter={ left, right } + rowGutter={ top, bottom } with support for single numbers or objects to avoid the breaking change.

[exogen]

@boygirl Should be ready!

[boygirl]

Thanks!