Removing `findDOMNode`

Hey @joshwcomeau. I look at #88 and saw [your comment](https://github.com/kdemoya/react-flip-move/blob/4ae798c45ca847daf7b33dc7465982f29aab7f35/src/dom-manipulation.js#L193-L195):

```
// While ReactDOM's `findDOMNode` is discouraged, it's the only
// publicly-exposed way to find the underlying DOM node for
// composite components.
```

I've been reading up on `findDOMNode`, there's a discussion at yannickcr/eslint-plugin-react#678. I thought I'd just leave a comment here detailing how one might remove use of this function. I'm not necessarily convinced this is a good idea.

First option is `FlipMove` would expect each child to be a html element, adding a `ref` with `cloneElement`. This means library consumers will have to wrap custom components in an `li` or `div`. `FlipMove` would just assume that all instances returned by `ref` are elements (erroring/warning otherwise).

Another option might be to require the custom child components to expose a `containerRef` prop.

```js
// `containerRef` prop required/supplied by `FlipMove`
function Custom({ containerRef, text }) {
return

function List({ items }) {
const itemElements = items.map(item =>

)
return {itemElements}
}
```

FlipMove can do something like:

```js
childrenWithRefs() {
return this.state.children.map(child => {
const ref = domNode => {
if (!this.childrenData[child.key]) {
this.childrenData[child.key] = {};
}
this.childrenData[child.key].domNode = domNode;
}
// If it's a html element, then we just use `ref`, otherwise require `containerRef`.
return isHtmlElement(child.type)
? React.cloneElement(child, { ref })
: React.cloneElement(child, { containerRef: ref })
})
}
```

Nice thing about this approach is that it means you can use stateless function components since they just have to pass the `ref` up via props.