Support passing fhirVersion as argument

[cmoesel]

In some cases, users might point GoFSH at a set of instances that don't declare they're FHIR version. In this case, it would be nice for GoFSH to allow the user to pass in the required FHIR version as an argument. Since -f and -v are already taken, perhaps it could be -u/--useFHIRVersion. E.g.:

gofsh /path/to/resources -u 5.0.0-snapshot1

See: https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/GoFSH.20and.20R5/near/274566527