Support for multiple versions of FHIR in a single IG

[cmoesel]

The IG community has been discussing how to support multiple versions of FHIR in the same IG. When the approach is determined, SUSHI should be updated to allow for whatever that approach is.

• See: https://confluence.hl7.org/display/FHIRI/FHIR+IG+version+support
• and: https://chat.fhir.org/#narrow/stream/179252-IG-creation/topic/Multi-FHIR.20version.20support/near/433372323
• and: https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/Developing.20R4.20and.20R5.20IGs.20in.20a.20single.20env.2E/near/426878828