Add support for React 16

`findDOMNode` is slated for deprecation soon, and support in React 16 appears to be partially dropped already.

For us to transition completely away from it, we would need to pass a ref prop down to the provided `content` component, including it as part of the `FlowTip` api. This is less ideal than automatically resolving the correct ref as `findDOMNode` currently does and would make the lib more complicated to use.

Until `findDOMNode` support is dropped completely, or we decide on a clean mechanism for passing the ref around, we rely on a custom `findDOMNOde` function that will safely attempt to detect the rendered node using react fiber internals when necessary. This universally supports React 16 and earlier versions.

(See: https://stackoverflow.com/questions/48062095/update-to-react-16-finddomnode-stops-working#answer-48273014)

Author: 10xjs