distill options passed into transformers

[boneskull]

Currently, we're sending whole gobs of configuration to transformers when we don't need to. Each transformer should probably define the options that it expects (e.g. via a schema like Rules support), and we can use that to pick the various useful options.

Configuration can be passed via:

1. Command-line flags (if supported)
2. Top-level via RC file
3. Command-specific via RC file
4. Transform-specific via RC file

Furthermore, default options should cascade within a transformer chain. If an option defines e.g., fields, and the user has not, this setting should pass to the next transformer(s) in the chain as long as those transformers accept the configuration. The target transformer should prefer the config sent from the previous transformer and treat it like it was user-defined.

[boneskull]

duplicate of #78 imo