Resolve #65: Add support for layer options in layer.yaml

[johnsca]

For #65. Add support for jsonschema defined and validated options in layer.yaml for configuring included layers.

[bcsaller]

It looks like in this case 'lint' is preforming a now needed mutation of the object which is a bit odd conceptually in that other tactics would be expected to do something similar. Can we do this in 'read' which should be related to loading the version of that data we expect to use in processing (maybe read should be 'load' or 'process' or 'parse' but that is another papercut).

[johnsca]

@bcsaller I agree that it's not ideal to do modification of the target values during validation, but that's unfortunately what the jsonschema library recommended. Ideally, the default setting behavior would be handled just after combine but I'm not clear on how to implement that without a lot of manual processing of the schema.