We could support [`transformer` plugins](https://meltano.com/docs/plugins.html#transformers) other than `dbt`, that would follow extraction rather than loading, and would transform an extractor's output stream of Singer messages (`SCHEMA`, `RECORD`, etc), before they're streamed into the loader. The two types of transformers could be distinguished using an extra, e.g. `type: {etl,elt}` or `{follows,acts_on,transforms}: {extractor,loader}` We could support both pip packages, and local executable files, using either the `pip_url` or `executable` plugin setting.