Schema flag for folders incorrectly treats "*.schema.json" files

When using the `--schema` option with a folder with schema files, quicktype will treat files with the `.schema.json` or `.json` extension like normal JSON files, not as schema files.

That is to say, it will generate code for `id`, `$schema` etc. properties in the JSON files, and not treat them as schema files.

We have only been able to work around this by naming the files with a ".schema" extension, which makes quicktype behave correctly, but which isn't the convention for naming JSON schema files, and is not supported by default in many editors.