Version control of external assets

[sebastianrath]

It would be great to explore the options of tracking external files/assets, besides the ones that are inside the repository. A project often consists out of a scene-file, assets in the project directory, but also assets outside of it.

Currently the Index converts all absolute paths into relative paths.

Proposal 1:
Extend index.addFiles(..., {external: true})

Proposal 2:
Extend index.addExternalFiles(...)