Use dynamic imports for loading STL and Collada Loader

[gkjohnson]

#80 moved over to use es6 imports but the STL and Collada loader will always be included in a build because they are included at the top of the file. The right way to handle this is to use the dynamic import() function.

This will implies code splitting for builds and adds complications for converting to a umd format because the imported file is external.

Rollup supports the --experimentalDynamicImport flag so that may be worth investigating.

[gkjohnson]

Related discussion here:

https://discourse.threejs.org/t/how-to-use-dracoloader-with-gltfloader-with-bare-module-imports/16748

[gkjohnson]

Rollup supports this, Parcel supports it, and Webpack will support it more cleanly in the next release.