New XML snippets for references

[ajeanmahoney]

The following would be helpful snippets to have available:

A snippet for a generic reference:

<reference anchor="CITATION-TAG" target="URL">
  <front>
    <title>TITLE</title>
    <author initials="" surname="" fullname="">
       <organization />
    </author>
    <date month="MONTH" year="YEAR"/>
  </front>
  <seriesInfo name="NAME" value="VALUE"/>
  <seriesInfo name="DOI" value="VALUE"/>
</reference>

And a snippet for an RFC. These are used when there isn't a bibxml entry for the RFC (i.e., when converting an I-D normative reference to an RFC one).

<reference anchor="RFCYYYY" target="https://www.rfc-editor.org/info/rfcYYYY">
  <front>
    <title>TITLE</title>
    <author initials="" surname="" fullname="">
      <organization/>
    </author>
    <date month="MONTH" year="YEAR"/>
  </front>
  <seriesInfo name="RFC" value="YYYY"/>
  <seriesInfo name="DOI" value="10.17487/RFCYYYY"/>
</reference>

[NGPixel]

Added both in 0.28.0