Port more object loaders across from THREE.js

[Floppy]

In my THREE.js code, as well as OBJLoader (which is already in mittsu), I use:

• STLLoader
• ThreeMFLoader
• PLYLoader

These are all currently in the "examples" section of the THREE.js code rather than the core, I'm not really sure why. If I port them to Ruby, would you be OK with them being in the core, or should I perhaps create a separate mittsu-extra-loaders gem?

[danini-the-panini]

I think it's separate from THREE.js to minimise the size of the JS, which isn't a big deal for a ruby gem, so I'm all for it being in core 👍🏻

[Floppy]

Nice one, I'll have a crack at one of them to get started, see how I get on.