reuse existing Three.js scene & renderer, given in options

Hi - I made this last week to allow use of Vizicities as a module/plugin in our multiuser WebTundra client framework.

Just allows passing in existing Three.js scene & renderer objects for Vizi to use so is completely generic in that way.

I'm not sure if this way of having a `threejs` object as an option like this is the best way, am open to suggestions and can refactor accordingly. This was a very simple change to make it working.

Where we use this now is at: https://github.com/playsign/fidemo/blob/master/main.js#L124

If this is or something like this is desirable to Vizi upstream, I/we can make an example or test to accompany the code change.