Bikeshed references parsing: also return informative references
Bikeshed references parsing: also return informative references

Structure returned now looks like:

{
  "normative": [],
  "informative": []
}

Note parsing reports an error when there are no normative references or no
informative references. We may want to be more flexible later on and report
warnings instead: specs may in theory not reference anything.

tidoust/reffy,tidoust/reffy