SUSHI crashes if you try to put extensions on pages in sushi-config.yaml

[cmoesel]

Apparently you can put FMM status on pages in an IG. If you try to do this, however, SUSHI crashes and prints out a usage statement.

To reproduce, add this in your sushi-config.yaml:

pages:
  index.md:
    title: Example Home
    extension:
      - url: http://hl7.org/fhir/StructureDefinition/structuredefinition-standards-status
        valueCode: trial-use
      - url: http://hl7.org/fhir/StructureDefinition/structuredefinition-fmm
        valueInteger: 3

If you run SUSHI, it will crash. If you delete or comment out the extension parts, it runs fine.

Expected behavior: Generate an IG JSON with those extensions at ImplementationGuide.definition.page[0].extension.

See: https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/FMM.20and.20standards.20status/near/284475473