Add option to keep `$ref`s (don't dereference `$ref`s)

[outdooracorn]

Thanks for creating this tool, it looks really useful.

It would be great to have an option that doesn't dereference $refs. There are some references, like ones that point to schemas in #/components/schemas, that I would like to keep in the single bundled/processed file.

The $refs should still be updated when multiple files are merged/bundled into one though. E.g. "$ref": "../../global/schemas.json#/ExampleSchema" should become "$ref": "#/components/schemas/ExampleSchema".