Output style for file-per-definition in ONE folder

The file-per-definition style currently splits definitions across folders.  A user has indicate that they would prefer they all be dumped into the same folder.  We should consider this as an additional style.

See: https://chat.fhir.org/#narrow/stream/215610-shorthand/topic/gofsh.20gotchas.20-.20watch.20out.20for.20file.20name.20clobbering