Add I-D identifiers, versioned and unversioned to `docid`

[ronaldtse]

As requested by https://github.com/ietf-ribose/relaton-data-ids/issues/15 , and relaton/relaton-iso#107

We want to have under docid two authoritative identifiers:

• Versioned identifier of an I-D: draft-xxx-nn
• Unersioned identifier of an I-D: draft-xxx

[andrew2net]

@ronaldtse should it be like:

  ...
  <docidentifier type="Internet-Draft" primary="true">draft--pale-email-00</docidentifier>
  <docidentifier type="IETF">draft--pale-email</docidentifier>
  ...