Add a possibility to specify default values for fields in schema salad

It would be interesting to be able to specify a default value for fields in schema_salad based specs. Currently they are mentioned in the documentation field, but this is not really machine readable (e.g. https://github.com/common-workflow-language/common-workflow-language/blob/master/v1.0/CommandLineTool.yml#L172). Having these would help a lot to generate SDKs ;)
cc @anton-khodak @khillion