Add URL for reporting issues.

Right now, for every project there's only one URL that points to the project's main site.

If possible, I would like the table to include these entries:

- An URL for issue reporting, if available.
- If the project is an exporter, the generator name prefix, ex: "babylon.js", "Sketchfab" , etc.

These two entries could be useful to add this functionality:

When an importer validates a glTF and finds errors, it can use the "generator" entry to find the project that produced the asset, and from there, report the Issue notification URL.

In other words, what I want to do is, when I load a glTF with errors, to be able to tell the user:

> The file xxxxx.gltf is malformed.
> Errors found: ....
> Warnings found ....
> Please, report these issues to http://www.somefancygltfexporter.com/issuereporting.html

Notice that, although many projects are reported as "exporters", under the hood they're using some other third party exporter, in these cases, I would either don't set the "generator", or I would point to the actual project that exports the asset as a dependency.