Configure ingestion pipeline from a configuration file

[ire-and-curses]

Currently the ingestion pipeline is built in code, e.g. like this.

It would be a big improvement if the description of a pipeline made up of the generic components we provide could be specified from a configuration file. The file should be a standard format (e.g. YAML) if possible. This would allow users to create custom pipelines without editing code, and also decouples user decisions from the code base, making it convenient to record or version control user's pipeline setups.