Update schema.json

[jogibear9988]

Add external Stylesheets

[jogibear9988]

fix for #116

[justinfagnani]

Thanks for the PR, and sorry it's taken me so long to review it.

Some notes on contributing:

1. Let's come to an agreement on the issue before making a PR
2. The PR title should be more descriptive than just "Update schema.json". Most PRs are going to update the schema in some way.
3. You need to make edits to schema.d.ts, then generate schema.json by running `npm run build'

On this feature, I'm not sure of the utility or who we actually specify this, so I'd like to close this until we have a clearer picture on #116