Add Defaultable for CommandInputRecordField

[tom-tan]

This request is re-done of common-workflow-language/cwl-v1.1#20 to fix common-workflow-language/common-workflow-language#641.
I introduced Defaultable and made it a parent record of InputParameter, WorkflowStepInput and InputRecordField.
I also add a conformance test for it.

I changed the description of Defaultable#default by combining descriptions of InputParameter#default and WorkflowStepInput#default.
Please let me know if there are better descriptions to unifying other descriptions of the default field.

Here is a list to be done (copied from common-workflow-language/cwl-v1.1#20):

• one or more conformance tests
• testing with cwltool --enable-dev in a v1.3.0-dev1 document
  • add this test permanently
  • Okay, need to teach cwltool how to copy the default value over
  • update the spec
• test cwltool using v1.0, v1.1 and v1.2 and a record field default (this should fail)
  • add this test permanently