support new citation markup

[pkra]

From AmerMathSoc/texml#183 (comment)

<cite-group>
    <x>[</x>
    <xref ref-type="bibr" rid="bibr-AEG0" specific-use="cite">AEG08<cite-detail><x>, </x>Section 5</cite-detail></xref>
    <x>; </x>
    <xref ref-type="bibr" rid="bibr-AEG0" specific-use="cite">AEG08</xref>
    <x>]</x>
</cite-group>

(Note: added whitespace just for clarity here.)

[pkra]

I think I might just copy them. They currently won't have attributes on them and (right now) I can't see a reason why they would ever get attributes.

[pkra]

By "them", I mean cite-group and cite-detail - both are valid (custom) html tag names.

[pkra]

I'm weirdly stumped by handling of x and how it will change (cf. AmerMathSoc/texml#183 (comment)).

Maybe it's time to move this hacky bit downstream now.

[pkra]

Since this is a breaking change either way, we might as well.

[pkra]

Ah, finally got it. I don't think this has to be a breaking change. But I've created #445 to remember.