Add ability to get component instance from react

[Alexander Buzin]

Currently, there is no way to get component object from react jsx structure. That means that we need to implement something like ref property, but for a component.

[Alexander Buzin]

Now each component has refComponent property.

Usage:

<BasicSphere key="3"
  position={[0, 6, 0]}
  refComponent={component => {
    component.material.color.setRGB(1, 1, 0); // Set yellow
  }}
/>