specify a URL to load scene from json file (v1)

[kfarr]

Here are the core user goals:

1. As a 3DStreet developer I want json-utils to exist only on 1 repo and be included in index.js
2. As an A-Frame developer I want to be able to specify a 3DStreet scene ID and/or scene URL in an A-Frame component so that I can include a 3DStreet scene in my A-Frame project.

[kfarr]

https://github.com/3DStreet/3dstreet/pull/352