CSS Custom Properties Should not be kebabified

Currently if a user tries to overwrite css custom properties using aphrodite the key will run through kebabify, and could possibily end up with the wrong value.

Example:
###In
```js
const styles = StyleSheet.create({
item: {
'--Some-Custom_Property': '#fff',
}
});
```
Out
```css
.item_someHash {
---some--custom_-property: #fff;
}
```

The issue lies in the `kebabifyStyleName` utility.

Start with the kebabifyStyleName utility identified in the issue and reproduce the shown custom-property example. Verify how the utility transforms names beginning with --, then confirm that CSS custom property names retain their intended spelling while regular style names are still kebabified.

Written by the indexing model from the issue text.