Support ES6 imports

[gkjohnson]

Provide an es6 import and UMD variant

[gkjohnson]

Use Parcel / Rollup to convert import to UMD

Edit --

Parcel may not work the way I want it to. Plan for now is to:

• convert the URDFLoader to a module that imports import * as THREE from 'three', which will require building to use.
• use Rollup to convert the file to a umd import that defines URDFLoader globally if just used as a script the way it does now.
• continue to use the umd module in the example file and use webpack to build
• the web components will only work with the umd module for now (will convert them in a future version)
• add a rollup watch command to the start script so it rebuilds the umd file.

[gkjohnson]

Closed via #80

#81 covers other possible work.