Add a mesh simplifier

[jgphilpott]

SimplifyModifier should do the trick: https://stackoverflow.com/a/41571483/1544937

Demo: https://threejs.org/examples/#webgl_modifier_simplifier
Code: https://github.com/mrdoob/three.js/blob/dev/examples/webgl_modifier_simplifier.html

[jgphilpott]

Partly done as of this commit: f033b6b45763b1dc37307fbaf58f209fe16719a5

The lib is now available but mesh methods should also be added.