Fix inline citation numbering

[shyamd]

This PR fixes inline citation numbering using the new system. A bug caused all citations to be labeled as 1. Thanks to @epogrebnyak for bringing this up in #107

[codecov-commenter]

Codecov Report

Merging #108 (d011c2d) into master (8f73346) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #108   +/-   ##
=======================================
  Coverage   96.18%   96.18%           
=======================================
  Files           3        3           
  Lines         131      131           
=======================================
  Hits          126      126           
  Misses          5        5           

Impacted Files	Coverage Δ
src/mkdocs_bibtex/plugin.py	98.27% <100.00%> (ø)

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