Refactor plugin for more modularity

[shyamd]

This PR is a major refactor of the plugin. It does have some breaking changes, so will likely be a major version update.

• Moves a lot of the functionality into functions in utils to make the logic easier to read
• Handle simple vs pandoc formatting with two different functions to keep them cleaner
• Update python package style and testing for future compatibility
• Manual unescape_for_arithmatex was removed in favor of always fixing this issue when using the pybtex backend. Pandoc doesn't seem to have this problem.

[codecov-commenter]

Codecov Report

Merging #104 (1f198ea) into master (7bf655e) will increase coverage by 3.25%.
The diff coverage is 97.60%.

@@            Coverage Diff             @@
##           master     #104      +/-   ##
==========================================
+ Coverage   92.92%   96.18%   +3.25%     
==========================================
  Files           2        3       +1     
  Lines          99      131      +32     
==========================================
+ Hits           92      126      +34     
+ Misses          7        5       -2     

Impacted Files	Coverage Δ
src/mkdocs_bibtex/__init__.py	66.66% <ø> (ø)
src/mkdocs_bibtex/utils.py	97.01% <97.01%> (ø)
src/mkdocs_bibtex/plugin.py	98.27% <98.27%> (ø)

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