Allow uploading of objects without adding to repo

[kfarr]

@Utopiah asks "City Builder could have access to another library of objects (maybe curated, at least with a specific scale) as an endpoint?"

Right now it is a cumbersome process to add new objects, I included a step-by-step guide on the updated readme here:
https://github.com/kfarr/aframe-city-builder/blob/master/README.md#how-do-i-add-new-objects-to-city-builder

It’s clear that the objects shouldn’t live in the repo in the long term. There's a firebase hosted version of this app at https://aframe.city and it is conceivable objects should be hosted file blobs and I was intending to move in that direction eventually. AND, if you want to be really fancy, there could be a cloud voxel editor for the objects. I’ve seen a few implementations of web based voxel editors, a few are quite fully featured.

[kfarr]

Useful asset library: http://kenney.nl/assets?q=3d

per https://twitter.com/utopiah/status/985873566297214981