Feature: allow to ignore types

[andre-brdoch]

Hey! A common scenario is that non-document schemas are defined in schema.js, like this:

export default createSchema({
  name: 'default',
  types: schemaTypes.concat(
    [
      // documents:
      article,
      category,

      // non-documents:
      link,
      video,
    ]
  ),
});

It would be great if those could be excluded from the graph, since they are effectively just fields, and clutter the graph 😄 If you want, I could make a PR for this 👍