Enable isDefinedBy links from formats (allow them to be empty and aggregate the info)

[bansp]

Enable the links usually commented out as, e.g., <relation target="SpecFLAC" type="isDefinedBy"/>

Allow them to point to standards documents but also de facto specifications, etc. That should be sorted out later in the process -- first, we need to actually have the pointers and demonstrate the need for doing something about them.