[RFC] disallow invalid step config options

Check if there are unexpected keys returned from the `loadStepsConfig` implementation to detect typos in option names.

But there might be use cases where `loadStepsConfig` is abused to add more information about steps, which is then used in userland code. This change would prevent individually extending the step config. Thus I'm not sure if custom options should be allowed or not.

Refs #91. /cc @jgornick