Progressively load assets on as-needed basis

[cvan]

In App.js all of the modules are imported at the same time, which consequently fires off a bunch of XHRs for models, audio, textures, etc - all the assets.

This results in a grand total of 193 HTTP requests at once. That enough XHRs on the main thread is enough to crash most low-end machines, especially mobile devices (which I understand is not our target); it's just something worth mentioning.

Culprit of issues #77 and #86. But, the main one was blocking on audio loading. This can come as an enhancement much later.

[cvan]

mrdoob/three.js#4248 (comment) has some good concepts.