Fix Pandoc

[shyamd]

This PR fixes a number of issues related to Pandoc

• Resolves pandoc-citeproc deprecated #75 by providing version-specific detection
• Tests against old and newer versions of pandoc to ensure this issue is easier to detect
• Should fix Citations being cut off #78

[codecov-commenter]

Codecov Report

Merging #82 (dddd8e3) into master (fdd881a) will increase coverage by 1.64%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #82      +/-   ##
==========================================
+ Coverage   90.90%   92.55%   +1.64%     
==========================================
  Files           2        2              
  Lines          99       94       -5     
==========================================
- Hits           90       87       -3     
+ Misses          9        7       -2     

Impacted Files	Coverage Δ
mkdocs_bibtex/plugin.py	94.31% <100.00%> (+1.84%)	⬆️

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