Add accessor for the extension field.

[mr-c]

Result of common-workflow-language/schema_salad#908

[coveralls]

coverage: 59.538% (-0.6%) from 60.167%
when pulling dddcd65 on extension_fields
into f35b0ba on cwl-1.2.0.

[kinow]

Looks good to me. Coverage check complaining… but that's a lot of getters to write tests for. Maybe we can proceed with the merge? Of maybe find a way to test everything at once (generate tests too? 😬 ).