Configurable breaks

[csala]

Make thematic breaks configurable with CLI arguments and .mdformat.toml options.

[codecov-commenter]

Codecov Report

Merging #2 (1b515f6) into master (5ef89d4) will decrease coverage by 38.09%.
The diff coverage is 51.51%.

@@             Coverage Diff              @@
##            master       #2       +/-   ##
============================================
- Coverage   100.00%   61.90%   -38.10%     
============================================
  Files            2        2               
  Lines           11       42       +31     
============================================
+ Hits            11       26       +15     
- Misses           0       16       +16     

Flag	Coverage Δ
pytests	61.90% <51.51%> (-38.10%)	⬇️

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Impacted Files	Coverage Δ
mdformat_simple_breaks/plugin.py	60.00% <50.00%> (-40.00%)	⬇️
mdformat_simple_breaks/__init__.py	100.00% <100.00%> (ø)

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