Keep position of citations with prefixes when sorting.
Keep position of citations with prefixes when sorting.

Closes #292.

If citations with prefixes are moved around, gibberish can
result.  E.g.:

    A fascinating research tidbit [@Zanadu1999, p. 35; see for comparison
    @Aalto2005] ...

can become

   A fascinating research tidbit (see for comparison Aalto, 2005;
   Zanadu, 1999: 35)

with a sorting style like sage-harvard.csl.

This change causes sorting to be applied only to subgroups of citations
within the group that lack prefixes.

adunning/pandoc-citeproc,jgm/pandoc-citeproc