[WIP] Add three plugin

[kdxcxs]

Introduction

It's a plugin makes three.js works with impress.js, which allows like 3d models to be added in impress.js canvas with all other feat from three.js.

This plugin is still in a very early stage, please do not merge. It's a cool feat for me, but I'm not sure if it's suitable for impess.js, so I opened this pr for further discution. The features are not fully implemented yet, but I've wrote a simple example in examples/three/index.html.

TODO

• Support to add 3d models by defining data sets.
• Add util functions to allow users to control the 3d models programmatically.

Issues

• As it's depend on three.js and tween.js, I'm not sure if it's ok for impress.js.
• How should plugin add the three.js canvas overlay into the dom?
  • It's currently implemented by adding a div#impress-three-container manually, maybe better to insert the canvas in plugin initialization with the root dom?
• As it's depend on three.js and tween.js, I'm not sure if it's ok for impress.js.
• The three.js overlay is always on the top of impress canvas, this could be wrong in position.
  • We could try creating a separate canvas for each 3d models and move with dom.
• The animation is not fully synced with css animation.

Ending

Here's still a lot not mentioned above, and I think here could be a lot changes to be made in impress.js, so I choose to contact you guys as early as possible to make it right. Or if this is plugin not what impress.js wanted, it's ok to reject and close this pr.