Support multiple versions of the validation schema

[hainesr]

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.