Support flag to require latest version of SUSHI

[cmoesel]

During the FHIR IG publication workflow, it is very important that the latest version of SUSHI be used. To support this, SUSHI should introduce an optional flag that indicates that SUSHI should exit with an error if it isn't the latest version (or if it can't determine the latest version). The default value should be false, but FHIR publication engineers can set the flag when doing a publication.

See: https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/FSH.20install.20problem/near/281720829