Support multiple versions of the validation schema #125
Description
As we move nearer to CFF version 1.3.0 this library needs to be able to validate CFF files against the version of the schema against which they have been written.
This is a non-trivial update as, in the true spirit of "maximise work not done," this library currently hard-codes validation against the 1.2.0 schema.
This issue is logged as a marker/reminder that this work needs to be done now.