Add support for :vertical and :horizontal pseudo classes

Hello,

I am trying to hide vertical scrollbar on Chrome with the following CSS rule `::-webkit-scrollbar:horizontal` according to [this documentation](https://webkit.org/blog/363/styling-scrollbars/).
Thus, I wrote an Aphrodite rule like this:
```javascript
rule: {
'::-webkit-scrollbar': {
':vertical': {
display: 'none',
}
}
}
```

But it does not work, if i just put the rule on '::-webkit-scrollbar', it works well on both vertical and horizontal scrollbars but the :vertical pseudo class seems to break.

No source file or test is named. Start by tracing how Aphrodite handles nested pseudo-selectors and compare its generated CSS with the issue's :vertical and :horizontal examples; done means both pseudo classes produce working scrollbar selectors.

Written by the indexing model from the issue text.