Citation-label formatting

Seems like the only sane place would be here:

<style label-format="Aaaa00:AaAA00:...">
</style>

With this to activate it for now:

  <features>
    <feature name="label-format" />
  </features>

It strikes me that mine won’t interpret characters that aren’t A/a/0 as literals to get the space in ABC 00, etc. Maybe allow literal A/a/0 with a backslash. It will put a year-suffix on the end, not in the code linked there but a label is a “year” for that purpose. Do people want anything else? Maybe people want the actual citekeys that they use? Not sure I’d ever want that, also probably unlikely in the Zotero community as default is autogenerated only, but maybe people still writing BibTeX would. Maybe people want to pull the citation-label variable from the input references as an override. I have actually implemented that already, it rings a bell as being in the test suite. That’s all I can think of.