Added InsertEdgeLoop

Added the classic "Insert Edge Loop" tool. 

I thought it would be the next best tool to add after extrude. Next, I might implement Cut/Knife tool, Bevel, or Mirror.

Right now every tool has been tested on the cube, which is manifold (meaning there's no boundary edges). I'm not looking forward to testing on non-manifold objects, and inevitably realizing that I need to add a significant amount code to every tool.