feat: Support ref to render props

[zombieJ]

添加 ref 代理以支持内部&外部同时获得 dom 节点：

• 使用 forwardRef （如果支持的话）将 ref 传递给 CSSMotion 处理
• CSSMotion 本身创建另一个 ref 给 render props 作为代理

const motionRef = React.createRef();

<CSSMotion
  ref={motionRef}
>
  {({ style, className }, ref) => (
    // 由于是 renderProps，需要用户自行把 ref 传递下去
    <div ref={ref} className={className} style={style} />
  )}
</CSSMotion>

[vercel]

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[coveralls]

Coverage increased (+0.2%) to 83.158% when pulling 05e44d5 on motion-ref into 5f1bf9b on master.