Using a placeholder does not work

If you pass in a placeholder prop, a ref will never be set so `this.visible` will always be false and LazyLoad will never render children.

See the following render method, where `setRef` is only called if placeholder is not defined: https://github.com/twobin/react-lazyload/blob/7411c5267d7019a418305b382f40220d20bdfba2/src/index.jsx#L294-L300

After some further investigation, it also looks like `ReactDom.findDOMNode` (which was used previously) was never _really_ working for custom placeholder components either--it seems to return a react element which fails the check here as it is not HTML: https://github.com/twobin/react-lazyload/blob/7411c5267d7019a418305b382f40220d20bdfba2/src/index.jsx#L126-L128